Index of underwater diving: N–Z

See the Glossary of underwater diving terminology for definitions of technical terms, jargon, diver slang and acronyms used in underwater diving

See the Outline of underwater diving for a hierararchical listing of underwater diving related articles

See the Index of underwater divers for an alphabetical listing of articles about underwater divers

See the Index of recreational dive sites for an alphabetical listing of articles about places which are recreational dive sites

The following index is provided as an overview of and topical guide to underwater diving:

Underwater diving can be described as all of the following:

• A human activity – intentional, purposive, conscious and subjectively meaningful sequence of actions. Underwater diving is practiced as part of an occupation, or for recreation, where the practitioner submerges below the surface of the water or other liquid for a period which may range between seconds to order of a day at a time, either exposed to the ambient pressure or isolated by a pressure resistant suit, to interact with the underwater environment for pleasure, competitive sport, or as a means to reach a work site for profit or in the pursuit of knowledge, and may use no equipment at all, or a wide range of equipment which may include breathing apparatus, environmental protective clothing, aids to vision, communication, propulsion, maneuverability, buoyancy and safety equipment, and tools for the task at hand.

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Na

• Namibian Marine Corps – Combat body created in 2016
• National Academy of Scuba Educators (NASE) – Recreational scuba training and certification agency
• National Association for Cave Diving (NACD) – American non-profit organization for improving cave diving safety
• National Association of Underwater Instructors (NAUI) – Non-profit training and certification agency association of scuba instructors
• National Board of Diving and Hyperbaric Medical Technology – Non-profit organization for education and certification in diving and hyperbaric medicine
• National Diving and Activity Centre – Flooded quarry in Gloucestershire used as a recreational dive site.
• National Navy UDT-SEAL Museum – Museum of US Navy UDTs and SEALs
• National Oceanic and Atmospheric Administration (NOAA) – US government scientific agency
• National Speleological Society Cave diving section – Organization for exploration, conservation, and study of caves in the United States
• NATO Submarine Rescue System – Project to develop an international submarine rescue system
• Nautical Archaeology Program – Graduate degree program at Texas A&M University
• Nautical Archaeology Society (NAS) – British organisation to further research in nautical archaeology for the public benefit
• Nautile – Crewed submersible owned by Ifremer
• Naval Air Command Sub Aqua Club – British organization within the Royal Navy
• Naval diving – Diving in navy applications
• Naval Diving Unit (Singapore) – Maritime special forces unit
• Naval Service Diving Section – Diving unit of the Irish Naval Service
• Naval Special Operations Command – Military unit of the Philippines Navy
• Naval Special Warfare Command (Thailand) – Special operations force within the Military of Thailand
• Naval Submarine Medical Research Laboratory – U.S. Navy research unit for submarine and diving medicine
• Navy diver (disambiguation) – Member of a country's naval forces, specializing in underwater diving
• Navy diver (United States Navy) – US Navy personnel qualified in underwater diving and salvage

Ne

• Neck dam – Flexible seal between a diving helmet and the diver's neck
• Necker Nymph – DeepFlight Merlin class positively-buoyant open-cockpit 3-seater wet sub
• Nederlandse Onderwatersport Bond (NOB) – Dutch governing body for underwater sports
• Negative buoyancy entry – Scuba diving water entry where the diver will immediately sink
• Negative pressure breathing – Breathing gas which is at a slightly lower pressure than in the resting lung
• Nemo 33 – Indoor recreational diving facility
• Nemrod – Spanish manufacturer of scuba and spearfishing equipment
• Neptune Finswimming Club – Club in Bristol, UK.
• Nereus (underwater vehicle) – Hybrid remotely operated or autonomous underwater vehicle
• Neutral buoyancy – Equilibrium between buoyancy and weight of an immersed object
• Neutral Buoyancy Laboratory – NASA astronaut training facility in Houston, Texas
• Neutral buoyancy pool – Pool of water in which neutral buoyancy is used to train astronauts
• Newtsuit – Atmospheric diving suit designed by Phil Nuytten
• New World Publications – Recreational dive guide publisher

Ni

• Night diver – Certification of competence to scuba dive at night
• Night diving – Underwater diving during the hours of darkness
• Nikonos – Brand of 35mm film amphibious cameras
• Nitrogen narcosis – Reversible narcotic effects of respiratory nitrogen at elevated partial pressures
• Nitrous oxide (medication) – Gas used as anesthetic and for pain relief
• Nitrox – Breathing gas, mixture of nitrogen and oxygen
• Nitrox blender – Person competent to blend nitrox for scuba diving
• Nitrox diver – Recreational diving qualification to dive using oxygen enriched air
• Nitrox production – Methods of producing nitrox mixtures

No

• NOAA Diving Manual – Training and operations manual for scientific diving
• abbreviation NDL – Maximum exposure without incurring decompression obligation
• NOGI Awards – Annual awards by Academy of Underwater Arts and Sciences.
• No-limits apnea – Freediving discipline in which the diver descends and ascends using their method of choice
• Nondestructive testing – Evaluating the properties of a material, component, or system without causing damage
• Non-freezing cold injury – Tissue injury due to sustained low temperature without freezing
• Nordic Deep – Freediving competition in Lysekil, Sweden
• Normocapnia – Normal arterial carbon dioxide levels
• Normoxic trimix diver – Recreational technical diving certification level
• Northern Ireland Federation of Sub-Aqua Clubs – National governing body for recreational diving and underwater sport in Northern Ireland
• Norwegian Diver School – Part of the Western Norway University of Applied Sciences
• No-stop limit – Diving bottom time beyond which obligatory decompression stops are incurred

Nu

• Nuclear diving – Diving in an environment where there is a risk of exposure to radioactive materials

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Ob

• Observation bell – Closed diving bell used as an observation platform

Oc

• Occupational diver – Person who dives underwater as part of their occupation
• Occupational diver training – Training of people employed as underwater divers
• Occupational diving – Underwater diving which is part of the job
• Occupational exposure limit – Upper limit on the acceptable concentration of a hazardous substance
• Occupational hazard – Hazard experienced in the workplace
• Occupational Health and Safety Act, 1993 – South African statutory law administered by the Department of Labour
• Occupational hygiene – Management of workplace health hazards
• Occupational safety and health, also known as Occupational health and safety – Field concerned with the safety, health and welfare of people at work
• Ocean current – Directional mass flow of oceanic water generated by external or internal forces
• Ocean Guardian – Personal electromagnetic field shark deterrent device
• Ocean stratification – Layering of ocean water due to density differences
• Oceanic Worldwide – American recreational scuba equipment manufacturer
• Octopus demand valve – Secondary demand valve on a scuba regulator
• Octopus regulator – Secondary demand valve on a scuba regulator

Of

• Offshore commercial diving – Professional diving in support of the oil and gas industry
• Offshore construction – Installation of structures and facilities in a marine environment
• Offshore diving – Diving outside the territorial waters of a country
• Offshore survey – Discipline of hydrographic survey largely concerned with the oil industry

Om

• Omitted decompression – Missed obligatory decompression stops
• Omitted decompression procedure – Procedure to reduce risk of developing decompression sickness

Op

• Open bell – Platform for lowering and lifting divers with an open-bottomed air space
• Open circuit breathing apparatus – Breathing apparatus where the exhaled gas is lost to the environment
• Open circuit demand helmet – Diving helmet which provides gas on inhalation and exhausts to the surroundings
• Open ocean diving – Diving in deep water out of sight of land
• Open water (diving) – Unrestricted water with free vertical access to the surface
• Open Water Diver – Entry-level autonomous diver certification for recreational scuba diving
• Open-water diving – Diving in unrestricted water when the diver has unresricted vertical access to the surface
• OpenROV – Open-source remotely operated underwater vehicle
• Operation Algeciras – Argentine plan to sabotage a British warship in Gibraltar
• Operation Source – Second World War Royal Navy midget submarine attacks on heavy German warships in Norway
• Operation Thunderhead – American amphibious mission during the Vietnam War
• Operational Diving Division (SA Navy) – Diving component of the South African Navy's Maritime Reaction Squadron
• Operations manual – Authoritative document of how things should be done in an organisation

Or

• Oral-nasal mask – Breathing mask that covers the mouth and the nose only.
• Orca Edge – First commercially viable personal decompression computer
• Orinasal mask – Breathing mask that covers the mouth and the nose only.

Os

• Osprey Reef – Submerged atoll in the Coral Sea, northeast of Queensland, Australia

Ou

• Outline of recreational dive sites – Hierarchical outline list of articles about rereational dive sites
• Outline of underwater divers – Hierarchical outline list of biographical articles about underwater divers
• Outline of underwater diving – Hierarchical outline list of articles related to underwater diving
• Out-of-gas emergency, also known as Out-of-air emergency – Underwater diving incident involving loss of breathing gas supply
• Out-of-gas incident, also known as out-of-air incident – Underwater diving incident involving loss of breathing gas supply

Ov

• Overfall – Dangerously steep and breaking seas due to currents over shallow obstructions
• Overconfidence effect – Personal cognitive bias
• Overhead environment, also known as overhead (diving) – Environment with a physical constraint to a direct vertical ascent to the surface
• Over-learning, also known as Overlearning – Practicing newly acquired skills beyond the point of initial mastery
• Overpressure valve (rebreather loop) (OPV) – Pressure relief valve on a breathing loop

Ox

• Oxygen analyser – Instrument to measure partial pressure of oxygen in a gas mixture
• Oxygen cleaning – Removal of contaminants that might cause a fire hazard in a n oxygen rich environment
• Oxygen compatibility – Usability in high-oxygen environments
• Oxygen enriched air – Air with added oxygen
• Oxygen fraction – The molar or volumetric fraction of oxygen in a gas
• Oxygen prebreathing – Procedure to reduce risk of decompression sickness before hypobaric exposure
• Oxygen service – Use in contact with high partial pressures of oxygen
• Oxygen therapy – Use of oxygen as a medical treatment
• Oxygen toxicity – Toxic effects of breathing oxygen at high partial pressures
• Oxygen toxicity seizure – Underwater diving emergency
• Oxygen window – Physiological effect of oxygen metabolism on the total dissolved gas concentration in venous blood
• Oxygen window in technical diving – Physiological effect of metabolism on total dissolved gas load in venous blood

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• P2 – Svenskt Vrakskydd – Organization for presrvation of shipwrecks in Swedish waters

Pa

• PADI Advanced Rebreather Diver – Minimum PADI certification meeting Rebreather Training Council standards
• PADI AWARE – Marine conservation non-profit organization for recreational divers
• PADI Delayed Surface Marker Buoy Diver – Diver trained to deploy a decompression buoy from depth
• PADI Discover Scuba Diving – Introductory scuba experience without certification
• PADI Low Impact Diver – Training to mitigate low entry level competence standards
• PADI Open Water Diver – Entry level recreational diving certification
• PADI Peak Performance Buoyancy – PADI diver with buoyancy control skill
• Panic – Sudden overwhelming sensation of fear
• Parachute lift bag – Open bottomed airtight bag used for underwater buoyant lifting when filled with air
• Partial pressure – Pressure of a component gas in a mixture
• Partial pressure blending – Gas blending based on pressure ratio of components
• Partial pressure vacancy – Physiological effect of oxygen metabolism on the total dissolved gas concentration in venous blood
• PASKAL – Special operations force of the Royal Malaysian Navy
• Passive addition semi-closed circuit rebreather, also known as passive addition rebreather – Dumps gas from the breathing circuit and makes up volume on demand
• Patent foramen ovale – A heart defect present at birth

Pe

• Pearl hunting – Collecting pearls from wild molluscs
• Pearling in Western Australia – Local aquaculture industry
• Pelagic Pressure Systems – Manufacturer of dive computers
• Pendulum rebreather – Configuration using biderctional gas flow
• Pendulum search – Diver search pattern
• Penetration dive – Dive under a physical barrier to a direct vertical ascent to the surface
• Penetration diving – Diving under a physical barrier to a direct vertical ascent to the surface
• Peress, Joseph Salim – Pioneering British diving engineer
• Performance Freediving International (PI) – Freediver training agency
• Perfusion – Passage of fluid through the circulatory or lymphatic system to an organ or tissue
• Permanent set – Plastic deformation
• Permeation – Penetration of a liquid, gas, or vapor through a solid
• Permit-to-work – Work safety management system
• Personal diving equipment – Underwater diving equipment worn by the diver
• Personal protective equipment – Equipment designed to help protect an individual from hazards

Ph

• Philippine Underwater Hockey Confederation – National sports association
• Phoenix International Holdings – Marine services company for underwater operations
• Physiological effects of the use of breathing apparatus
• Physiological response to water immersion – Also known as the diving response and mammalian diving reflex
• Physiology of decompression – The physiological basis for decompression theory and practice
• Physiology of diving adaptations – Adaptations of marine vertebrates to diving
• Physiology of human diving – Influences of the underwater environment on the physiology of human divers

Pi

• Pillar valve represented by Scuba cylinder valve – Valve controlling flow of breathing gas into and out of a scuba cylinder
• Piccard, Auguste – Swiss physicist, inventor, and explorer
• Pinch point hazard – Mechanical hazard which may damage or injure by reducing a gap
• Pipeline end manifold – Termination of a rigid pipeline

Pn

• Pneumofathometer – Instrument for measuring the depth of a diver by air pressure

Po

• Police diving – A branch of professional diving carried out by police services
• Polespear – Basic rubber launched underwater fishing spear
• Police diving – A branch of professional diving carried out by police services
• Neal W. Pollock – Canadian researcher in diving physiology and hyperbaric medicine
• Pony bottle – Small independent scuba cylinder usually carried for emergency gas supply
• Pool part of the 2011 Finswimming World Championships – International competition in Hódmezővásárhely, Hungary
• Pool part of the 2013 Finswimming World Championships – International competition in Kazan, Russia
• Porpoise (rebreather) – Australian oxygen rebreather
• Porpoise (scuba gear) – Australian scuba manufacturer
• Powerhead – Specialized firearm used underwater that is fired when in direct contact with the target
• Positive pressure breathing – Breathing gas which is at a slightly higher pressure than in the resting lung
• Positive pressure breathing apparatus – Equipment in which pressure insude the facepiece is always higher than ambient pressure
• Positive pressure open circuit breathing apparatus – Apparatus supplying gas at a pressure slightly above ambient
• Potable water diver – Person competent to dive in potable water systems
• Potable water diving – Underwater diving in potable water systems
• Power inflator – Low pressure gas supply valve for buoyancy compensator
• PowerSwim – Diver-powered propulsion device

Pr

• Pre-dive briefing – Meeting of the dive team to discuss details before the diving operation
• Pre-dive checks – Checks done prior to entering the water for a dive
• Preparing the diving rebreather – Assembly and predive function tests
• Pressure – Force distributed over an area
• Pressure reducing regulator – Pressure control valve that maintains the downstream pressure setting
• Pressure reduction regulator – Control valve that reduces and maintains the outlet pressure of a fluid
• Pressure swing adsorption – Method of gases separation using selective adsorption under pressure
• Pressure-tight zipper – Zip fastener capable of a seal across a gas pressure difference
• Pressure transmitter (diving) – Cylinder pressure transducer with wireless transmitter linked to dive computer
• Pressure vessel for human occupancy, also known as pressure vessel for human occupation – Container to be occupied by one or more people with an internal pressure different from outside
• Primary demand valve – The second stage regulator that the diver normally uses
• Priz-class deep-submergence rescue vehicle – Russian submersibles
• Professional Association of Diving Instructors (PADI) – Recreational diver training and certification agency
• Professional Diver's Handbook – John Bevan Ed. A manual of offshore diving
• Professional Divers' Association of Australia – Trade union
• Professional diver training – Training of candidates for registration as professional divers
• Professional diving, also known as Occupational diving – Underwater diving where divers are paid for their work
• Professional Diving Instructors Corporation (PDIC) – Recreational diver training and certification agency
• Professional scuba diver – Person with skills and knowledge required for occupational diving
• Professional Technical and Recreational Diving – Diver certification agency
• Protection of Military Remains Act 1986 – UK legislation protecting wreckage of military ships and aircraft
• Protection of Wrecks Act 1973 – UK legislation on conservation of shipwrecks

Ps

• Psychological fitness to dive – Psychological fitness of a person to function safely underwater under pressure
• Psychosocial hazard – Occupational hazard to one's psychological well-being
• Psychrometric constant – Relation of the partial pressure of water in air to temperature

Pu

• Public safety diver – Diver working in the public safety sector
• Public safety diver training – Training divers for public safety services
• Public safety diving – Underwater work done by law enforcement, rescue and search and recovery teams
• Pulmonary barotrauma – Lung over-pressure and squeeze injuries
• Pulmonary circulation – Part of the circulatory system which carries blood from heart to lungs and back to the heart
• Pulmonary edema of immersion – Body fluid in the lungs while immersed
• Pulmonary oxygen toxicity – Lung injury due to nlong term exposure to high oxygen levels
• Pulmonary volutrauma – Lung injury due to over-expansion
• Purge button – Manual activation control of the demand valve

Py

• Pyle stop – Type of short deep decompression stops in addition to the standard profile
• Pyle, Richard – American ichthyologist and scuba diver

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• Quintana Roo Speleological Survey – Data repository for explored sites within the state of Quintana Roo

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• R-2 Mala-class swimmer delivery vehicle – 2-man wet sub swimmer delivery vehicle class of the Yugoslavian, and later, Croatian, Navies

Ra

• Rack operator – Member of a surface-supplied diving team who operates the breathing gas panel
• Raid on Alexandria (1941) – Italian frogman raid on British warships
• Raid on Algiers – Italian frogman raid on Allied ships in Algiers harbour in 1942
• Rash guard, also known as Rash vest – Stretch garment for protection from abrasion, UV and stings
• Ratchet reel (diving) – Type of reel used by divers to deploy and recover line
• Ratio deco – Rule of thumb for estimating a decompression schedule for a given set of breathing gases
• Ratio decompression – Rule of thumb for estimating a decompression schedule for a given set of breathing gases
• Rawlins, John – Royal Navy officer and pioneer in the field of diving medicine

Re

• Rebreather – Portable apparatus to recycle breathing gas
• Rebreather ascents and descents – Loop volume and partial pressure control with changes in ambient pressure
• Rebreather Association of International Divers (RAID) – Recreational diver training and certification agency
• Rebreather bailout – Switching over to an alternative supply of breathing gas
• Rebreather diluent flush, also known as diluent flush, or dil flush – Replacing gas in the loop with fresh diluent
• Rebreather diver – Scuba diving qualification to use a rebreather
• Rebreather diving – Underwater diving using self contained breathing gas recycling apparatus
• Rebreather Education and Safety Association (RESA) – Organisation of rebreather manufacturers
• Rebreather endurance – The duration that a rebreather can adequately supply appropriate breathing gas
• Rebreather head-up display – User information interface always in view of the diver
• Rebreather loop – Ambient pressure breathing gas circuit
• Rebreather scrubber – Component that removes carbon dioxide from exhaled gas
• Rebreather Training Council (RTC) – Organisation to coordinate rebreather training standards
• Reclaim helmet – Diving helmet that returns exhaled gas through a hose for recycling
• Reclaim regulator – Diving regulator controlling flow of exhaled gas into low pressure exhaust return hose
• Reclaim valve – Backpressure regulator controlled relative to ambient pressure for breathing gas recovery
• Recompression (disambiguation) – Returning to a state of higher ambient pressure
• Recompression chamber – A hyperbaric chamber used to treat divers suffering from decompression illness
• Recovery breathing – Technique used by free divers on surfacing to reduce the risk of surface blackout.
• Recovery of a convulsing diver – Procedure for rescue of a convulsing scuba diver from underwater
• Recovery of an unresponsive diver – Procedure for rescue of a convulsing casualty from underwater
• Recreational dive – Underwater dive done for recreational purposes
• Recreational Dive Planner – PADI no-decompression dive table also available as a circular slide rule and electronic calculator
• Recreational dive sites – Places that divers go to enjoy the underwater environment
• Recreational diver certification – Certification as competent to dive to a specified standard
• Recreational diver certification agency – Agencies which issue certification for competence in recreational diving skills
• Recreational diver impact on reefs – Effects of scuba diving on the underwater environment
• Recreational diver training – Training process for people who do not dive at work
• Recreational divers – People who engage in underwater diving for entertainment
• Recreational diving – Diving for the purpose of leisure and enjoyment, usually when using scuba equipment
• Recreational diving fatalities – Deaths occurring during scuba dives
• Recreational diving instructor – Person who trains and assesses recreational divers
• Recreational diving service provider – Tangible and intangible goods and services provision for recreational diving tourism
• Recreational diving tourism – Industry based on recreational diver travel
• Recreational scuba certification levels – Competence represented by recreational scuba certification
• Recreational scuba diving – Diving for leisure and enjoyment while using scuba equipment
• Recreational trimix diver – Certification to dive with normoxic trimix with equivalent narcotic depth 80 ft or less
• Reduced gradient bubble model – Decompression algorithm
• Redundancy (engineering) – Duplication of critical components to increase reliability of a system
• Redundant breathing gas supply – Scuba breathing gas reserved for an emergency
• Reef – A shoal of rock, coral or other sufficiently coherent material, lying beneath the surface of water
• Reef Ball Foundation – Non-profit organization to promote artificial reef building
• Reef Check – International NGO for reef conservation
• Reef Life Survey (RLS) – Marine life monitoring programme based in Hobart, Tasmania
• Refresher course – Training to update existing skills and knowledge
• Refresher training – Training to update existing skills and knowledge
• Refresher training (diving) – Checkout of skills or remedial training to bring skills back up to standard
• Regulator freeze – Underwater breathing apparatus malfunction
• Regulator freezing – Locking of a diving regulator by ice formation
• Regulator icing – Locking of a diving regulator by ice formation
• Regulator malfunction – Failure modes for diving regulators
• Regulator necklace – Safety item for scuba diving, particularly with a rebreather
• Remotely operated underwater vehicle (ROUV or ROV) – A tethered underwater mobile device operated by a remote crew
• REMUS (AUV) – Autonomous underwater vehicle series
• Repetitive dive – Dive made while outgassing from a previous dive is incomplete
• Repex – Repeated exposure to potentially toxic concentrations of oxygen in breathing gas
• Rescue bell – Diving bell for submarine rescue
• Rescue diver – Recreational scuba certification emphasising emergency response and diver rescue
• Rescue Diver – Recreational scuba certification emphasising emergency response and diver rescue
• Rescue sling – Device used for lifting a casualty
• Rescue tether – Equipment to facilitate a diver rescue
• Rescue ventilation – Assisted breathing to support life
• Reserve gas supply (diving) – Surface breathing gas supply for diving use if main supply is compromised
• Reserve valve – Valve isolating access to reserve gas supply
• Residual inert gas tissue loading – Gas remaining in the tissues after decompression
• Residual nitrogen – Excess nitrogen remaining in body tissues after surfacing from a dive
• Residual nitrogen time – Estimate of excess nitrogen remaining in tissues before repetitive dive
• Resort diving – Introductory scuba diving experience
• Respiration (physiology) – Exchange of gases between environment and tissues
• Respiratory exchange ratio – Ratio between the metabolic production of carbon dioxide and the uptake of oxygen
• Respiratory gas exchange – Process by which gases diffuse through a biological membrane
• Respiratory interface – Facepiece of a breathing apparatus
• Respiratory quotient – Ratio of carbon dioxide produced by the body to oxygen consumed by the body
• Restriction (diving) – Space through which it is possible to pass with some difficulty
• Reverse squeeze – Failure of the Eustachian tubes to open during a reduction in ambient pressure

Ri

• Rip current – Water current moving away from shore
• Risk assessment – Estimation of risk associated with exposure to a given set of hazards
• Risk control – Process in which identified risks are reduced or mitigated
• Risk management – Identification, evaluation and control of risks
• RN Diving Manual – Training and operations manual

Ro

• Rock bottom gas planning – Scuba gas planning based on a planned dive profile
• Rocky reef – Natural reef of rock
• Röjdykare – Clearance divers of the Swedish Navy
• ROUV – Remotely operated underwater vehicle
• ROV KIEL 6000 – Remotely operated vehicle built by Schilling Robotics, Davis, California for scientific tasks
• ROV PHOCA – Remotely operated underwater vehicle of the COMANCHE type
• ROV Supervisor – A senior ROV pilot appointed to supervise the ROV team
• Royal Australian Navy School of Underwater Medicine (RANSUM) – Unit based in Sydney, Australia.
• Royal Engineers – Engineering arm of the British Army
• HMS Royal George (1756)#Salvage attempts – 100-gun first-rate Royal Navy ship of the line (1756) – Early salvage operation using bells and surface supplied divers
• Royal Navy ships diver – Category of underwater diver in the Royal Navy

Ru

• Rubicon Foundation – Non-profit organization for promoting research and information access for underwater diving
• Rule of thirds (diving) – Rule of thumb for scuba gas management
• Russian commando frogmen – Tactical scuba diving unit
• Russian deep submergence rescue vehicle AS-28 – Priz-class deep-submergence rescue vehicle of the Russian Navy
• Russian submarine AS-34 – Priz-class deep-submergence rescue vehicle
• Russian submarine Losharik – Russian deep-water nuclear submarine

S

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Sa

• Safety culture – Risk-averse attitudes
• Safety data sheet, also known as Material safety data sheet – Sheet listing work-related hazards
• Safety sausage – Inflatable buoy used by a diver at the surface to indicate position to the dive boat
• Safety stop – Optional decompression stop to reduce decompression stress
• Salt spray aspiration – Inhalation of salt water droplets
• Salt water aspiration syndrome – Rare diving disorder caused by inhaling a mist of seawater
• Salvage diving – Diving work associated with the recovery of vehicles, cargo and structures
• Salvage of the SS Egypt's gold – Recovery of bullion from wreck using an atmospheric pressure observation bell
• Salvage operations on HMS Royal George – Early salvage operations using bells and surface supplied divers
• Sappers Divers Group – Portuguese Navy's diving unit
• Saturation diver – Diver registered as competent for saturation diving
• Saturation diving – Diving decompression technique
• Saturation diving skills – Surface-supplied diving skills specific to saturation operations
• Saturation diving system – Facility for supporting saturation diving projects
• Saturation spread – The topside base for saturation diving operations
• Saturation system – Diving decompression system
• Save Ontario Shipwrecks (SOS) – A provincial heritage NGO in Ontario, Canada
• S-BRUV – Stereoscopic baited remote underwater video

Sc

• Science of underwater diving – Scientific concepts that are closely associated with underwater diving
• Scientific diver training – Training divers who will be doing scientific work underwater
• Scientific diving – Use of diving techniques in the pursuit of scientific knowledge
• SCMBA – British military rebreather
• Scorpio ROV – Work class remotely operated underwater vehicle
• Scottish Sub Aqua Club (ScotSAC) – Scottish recreational diver training and certification agency
• Screw gate carabiner – Carabiner snap hook with screw lock gate
• Scrubber breakthrough – Failure mode in rebreathers when the carbon dioxide scrubber is exhausted
• Scrubber endurance – The duration that a rebreather scrubber can adequately remove carbon dioxide
• Scuba configuration – The arrangement of self contained breathing apparatus carried by a diver
• Scuba contents gauge – Submersible pressure gauge for scuba cylinders
• Scuba cylinder valve – Valve controlling flow of breathing gas into and out of a scuba cylinder
• Scuba diving – Swimming underwater, breathing gas carried by the diver
• Scuba diving fatalities – Deaths occurring while scuba diving or as a consequence of scuba diving
• Scuba diving in the Cayman Islands – Recreational diving tourism destination
• Scuba Diving International (SDI) – Recreational diver training and certification agency
• Scuba diving quarry – Disused and flooded quarry repurposed for underwater diving
• Scuba diving skills – The skills required to dive safely using self-contained underwater breathing apparatus
• Scuba diving therapy – Treatment using scuba diving activities
• Scuba diving tourism – Industry based on recreational diver travel
• Scuba Educators International (SEI) – Recreational diver training and certification agency
• Scuba emergency procedures, also known as Scuba emergency skills – Standard procedures for use in common scuba diving emergencies
• Scuba equipment – Equipment needed for scuba diving
• Scuba fatalities – Deaths occurring while scuba diving or as a consequence of scuba diving
• Scuba finswimming – Underwater sport discipline
• Scuba gas blender – Person competent to blend breathing gases for scuba diving
• Scuba gas consumption – Breathing gas used while scuba diving
• Scuba gas management – Logistical aspects of scuba breathing gas
• Scuba gas planning – Estimation of breathing gas mixtures and quantities required for a planned dive profile
• Scuba gas reserve – Breathing gas reserved for contingencies
• Scuba gear – The equipment used by a scuba diver for diving
• Scuba harness – Equipment for supporting a scuba set on a diver
• Scuba isolation manifold – Component used to functionally connect scuba cylinders
• Scuba manifold – Scuba component used to functionally connect diving cylinders
• Scuba mouthpiece – Bite grip breathing interface on demand valve
• Scuba orienteering – Underwater compass navigation and speed competition on scuba.
• Scuba procedures – Standardised methods for scuba diving activities
• Scuba refresher course – Coaching to restore skills and knowledge
• Scuba regulator – Mechanism that controls the pressure of a breathing gas supply for scuba diving
• Scuba replacement – Surface-supplied diving mode using only gas supplied from high pressure cylinders
• Scuba reserve valve – Scuba gas supply valve which is opened to release the gas held in reserve
• Scuba Schools International (SSI) – Recreational scuba and freediving training and certification agency
• Scuba set – Self-contained underwater breathing apparatus
• Scuba skills – The skills required to dive safely using a self-contained underwater breathing apparatus.
• Scubapro – Brand of scuba diving equipment
• SDBA – Special duty oxygen breathing apparatus, a military rebreather.

Se

• Seabed mining – Mineral recovery from the bottom of the sea
• Seabed tractor – Special purpose class of remotely operated underwater vehicle
• Sea Dragon-class ROV – Chinese deep diving work class remotely operated underwater vehicle
• Seafox drone – Remotely operated anti-mine marine drone
• Seaglider – Deep diving autonomous underwater vehicle for long term missions
• SeaKeys – Collaborative marine biodiversity project in South Africa
• SEALAB – Experimental underwater habitats developed by the United States Navy
• SEAL Delivery Vehicle – Manned wet submersible for deploying naval special forces
• Sea level atmospheric pressure – Static pressure exerted by the weight of the atmosphere at sea level
• SeaPerch – Remotely operated underwater vehicle educational program
• Sea Pole-class bathyscaphe – Chinese bathyscaphe class
• Sea Research Society – American nonprofit for marine research
• SEAL Delivery Vehicle (SDV) – Manned wet submersible for deploying naval special forces
• Search and recovery diver – Diver certified as trained in underwater search and recovery
• Willard Franklyn Searle – US Navy ocean engineer and developer of diving and salvage equipment and systems
• Seasickness, also known as Motion sickness – Motion sickness occurring at sea
• Secondary regulator second stage – Second demand valve on a scuba regulator first stage
• Self-reliant diving – Philosophy and practice of diving that does not rely on assistance
• Self-sufficient diving – Scuba diving with skill and equipment to manage most contingencies without assistance
• Semi-submersible platforms – Marine vessel used in offshore roles wtth good stability and seakeeping
• Sentry (AUV) – Autonomous underwater vehicle made by Woods Hole Oceanographic institution
• Sewer diving – Diving for maintenance work in sewers

Sh

• Shadow Divers – Book by Robert Kurson recounting the discovery of a World War II German U-boat wreck
• Shallow water blackout – Loss of consciousness at a shallow depth during a dive
• Shallow Water Combat Submersible – Manned submersible and a type of swimmer delivery vehicle
• Shallow water helmet – Free-flow diving helmet for shallow work
• Shallow-water blackout – Loss of consciousness at a shallow depth during a dive
• Shallow-water diving helmet – Open-bottom diving helmet held on by weight
• Shark baiting – Attracting sharks by chumming the water
• Shark cage diving – Diving inside a protective cage to observe sharks in the wild
• Shark-proof cage – A metal structure to protect divers and snorkellers from potentially dangerous sharks
• Shark River Reef – Artificial reef off New Jersey
• Shark tourism – Tourism industry based on viewing wild sharks
• Shayetet 13 – Special operations unit of the Israeli Navy
• Shearwater Research – Canadian manufacturer of dive computers and rebreather electronics.
• Shilling, Charles Wesley – U.S. Navy physician, researcher, and educator
• Ships husbandry – Maintenance and upkeep of ships
• Ships husbandry diving – Diving related to the maintenance and upkeep of ships
• Shot-line, also known as shot line or shotline – Substantial weighted near-vertical line with buoy

Si

• Sidemount – Diving equipment configuration where scuba sets are clipped to the diver's sides
• Sidemount diver – Scuba certification for using scuba sets clipped to the sides of the harness
• Sidemount diving – Diving using equipment configuration where scuba sets are clipped to the diver's sides
• Side-mount scuba – Scuba sets carried clipped to the sides of the diver's harness
• Side-slung bailout set – Bailout cylinder clipped to the side of the diver's back-mount cylinder harness
• Siebe, Augustus – British engineer mostly known for his contributions to diving equipment
• Siebe Gorman – British manufacturer of diving equipment and salvage contractor
• Siebe Gorman CDBA – Type of diving rebreather used by the Royal Navy
• Siebe Gorman Salvus – Industrial rescue and shallow water oxygen rebreather
• Signal tube – Inflatable buoy used by a diver at the surface to indicate position to the dive boat
• List of signs and symptoms of diving disorders – Evidence of physiological disorders resulting from underwater diving
• Silt out – Reduction of underwater visibility by disturbing silt deposits
• Silt out, also known as siltout, or silt-out – Reduction of underwater visibility by disturbing silt deposits
• Silt screw – A peg for insertion into soft sediment to locate a distance line
• Single atmosphere diving suit – Articulated pressure resistant anthropomorphic housing for an underwater diver
• Single-hose diving regulator, also known as single hose – Regulator with two stages connected by a low-ressure hose
• Single point of failure – A part whose failure will disrupt the entire system
• The Silent World: A Story of Undersea Discovery and Adventure – Book by Jacques-Yves Cousteau and Frédéric Dumas
• Siluro San Bartolomeo – Italian manned torpedo design of late WWII
• Single point of failure – A part whose failure will disrupt the entire system
• Sinking of MV Conception – 2019 maritime disaster
• Sinking of the Rainbow Warrior – 1985 covert attack by French foreign intelligence service on a Greenpeace ship
• Sinking ships for wreck diving sites – Scuttling old ships to produce artificial reefs
• Sinus squeeze – Barotrauma of the sinuses
• Sistema Huautla – Cave system in the southern Mexican state of Oaxaca
• Sistema Nohoch Nah Chich – Flooded cave system in Mexico
• Sistema Ox Bel Ha – Flooded cave system in Quintana Roo, Mexico
• Situation awareness – Adequate perception of environmental elements and external events
• Siva (rebreather) – Range of military rebreathers
• SJT-class ROUV – Series of Chinese remotely operated underwater vehicles

Sk

• Skandalopetra diving – Freediving using a stone weight at the end of a rope to the surface
• Skin bend – Decompression symptoms in superficial tissues
• Skindiving (disambiguation)

Sl

• Sladen Suit – Early British military drysuit
• Sling (rigging) – Rope, webbing, wire or chain used to support a load for lifting
• Sling cylinder – An independent scuba set carried clipped to the side rings of a scuba diver's harness
• Slingshot valve – Dual outlet scuba cylinder valve with Y-shape body

Sm

• Smith, Gordon – Inventor of KISS diving rebreather, born 1950

Sn

• Snag-line search – Use of a taut line towed across the search area to catch on the target
• Snell's law, also known as Law of refraction – Formula for refraction angles
• Snoopy loop, also known as ranger band – Rubber band made from inner tube
• Snorkel – Tube for breathing face down at the surface of the water
• Snorkel mask – Breathing apparatus for surface swimming
• Snorkeling – Swimming while inhaling through a snorkel
• Snorkeling vest, also known as snorkelling vest – Peronal buoyancy aid for use while snorkeling
• Snuba – Limited depth airline breathing apparatus towed by the diver

So

• Society for Underwater Historical Research – Amateur maritime archaeology organisation in South Australia
• Solo diver – SDI recreational diver self-sufficiency certification
• Solo diving – Recreational diving without a dive buddy
• Solo diving skills – Addition scuba skills of self-reliance
• Solubility – Capacity of a substance to dissolve in a homogeneous way
• Solution – Homogeneous mixture of a solute and a solvent
• Sonar – Acoustic sensing method
• Sonic orifice – Orifice undergoing choked flow of a compressible medium
• South African Association for Marine Biological Research (SAAMBR) – Non profit conservation research organisation based in Durban, SA
• South African Department of Employment and Labour – Department of the South African government responsible for matters related to employment
• South African Environmental Observation Network (SAEON) – A network to perform long-term ecological research in South Africa and surrounding waters
• South African Underwater Sports Federation (SAUSF) – Official World Underwater Federation representative body in the Republic of South Africa
• South Pacific Underwater Medicine Society (SPUMS) – Publisher for diving and hyperbaric medicine and physiology
• South Pacific Underwater Medical Society Journal – Journal covering underwater and hyperbaric medicine and physiology
• Southern African Underwater and Hyperbaric Medical Association (SAUHMA) – Special interest group of the Council of the South African Medical Association

Sp

• SP-350 Denise – French two-person submarine
• Spearfisherman (company) – American diving equipment manufacturer
• Spearfishing – Hunting for fish using a spear
• Speargun – Underwater fishing implement
• Special Actions Detachment – Special operations maritime unit of the Portuguese Navy
• Special Air Service – Special forces of the British Army
• Special Air Service Regiment – Special forces unit of the Australian Army
• Special Boat Service – Special forces unit of the Royal Navy
• Special Duties Unit – Hong Kong Police tactical unit
• Special Forces Command (Turkey) – Turkish special forces group
• Special Forces Group (Belgium) – Special forces unit in the Belgian Armed Forces
• Special Operations Battalion (Croatia) – Special forces military unit
• Special Service Group (Navy) – The special operations force of the Pakistan Navy
• Special Warfare Diving and Salvage – Bangladesh Navy special operations force
• Speleonaut – Diver propulsion vehicle designed for cave exploration by a disabled diver
• Spindle (vehicle) – Ice penetrating two-stage autonomous underwater vehicle
• Spiral box search – Search pattern expanding stepwise outward from datum
• Spitcock – Valve on diving helmet to let diver spit water on inside of viewport
• Sponge diving – Diving to gather natural sponges
• Sport (shipwreck) – Tugboat wrecked in Lake Huron
• Sport diving (sport) – Underwater sport using recreational open circuit scuba equipment in a swimming pool
• SPP-1 underwater pistol – Soviet four-barreled underwater dart pistol
• SPUMS – Publisher for diving and hyperbaric medicine and physiology
• SPURV – Self propelled underwater research vehicle built in 1957 for the US Navy
• SPURV II – Special purpose underwater research vessel built to srudy submarine wakes
• Squeeze (diving) – Barotrauma of descent
• SRV-300 – Deep-submergence rescue vehicle

St

• Stabilizer jacket – Configuration of diving buoyancy compensator
• Stage cylinder – Diving cylinder intended to be collected for use on the return part of a scuba dive
• Staged deccompression, also known as Stage decompression – Decompression with stops ad specific depths
• Stage drop, also known as Stage-drop – Placing a scuba cylinder at the distance line for planned later use.
• Stage rigging (scuba) – Clips, cords and straps on a scuba cylinder to attach it to a diver
• Stage set (scuba) – Scuba set to be used during a stage of a dive
• Staged decompression – Ascant from dive interrupted by periods at constant depth
• Standard diving dress – Copper helmet with rubberised canvas diving suit and weighted boots
• Standard diving equipment – Copper helmet with rubberised canvas diving suit and weighted boots
• Standard diving helmet – Copper free-flow helmet system used with Standard Diving Dress
• Standard helmet – Helmet of standard diving dress
• Standard operating procedure – Set of detailed instructions to assist in workplace safety
• Standard procedure – A recommend way to do something that is known to be effective and safe
• Standby diver, also known as Stand-by diver – Diver who is ready to go to the assistance of the working diver
• Standard diving dress – Copper helmet with rubberised canvas diving suit and weighted boots
• Standard operating procedure – Set of detailed instructions to assist in workplace safety
• Star Canopus diving accident – Fatal offshore diving bell accident in 1978
• Static apnea – Stationary diving discipline of holding breath underwater
• Stena Seaspread diving accident – Saturation diving bell incident with successful rescue in the North Sea in 1981
• Stratification – Layering of a body of water due to density variations
• Stress exposure training – Training in a deliberately distracting environment
• Striver (bathyscaphe) – Chinese deep submergence vehicle
• Struggler (bathyscaphe) – Chinese deep submergence vehicle

Su

• Sub-Aqua Association (SAA) – British recreational diver training and certification organisation
• Submarine emergency gas supply – Breathing gas system installed in submarine for emergency use
• Submarine Escape Immersion Equipment – Whole-body exposure suit that allows submariners to escape from a sunken submarine
• Submarine escape set – Self contained breathing apparatus providing gas to escape from a submerged submarine
• Submarine escape training facility – Facility used for training submariners in methods of escape from a sunken submarine
• Submarine Escape Training Facility (Australia) – Shore facility of the Royal Australian Navy
• Sub Marine Explorer – Early submarine craft
• Submarine pipeline – Pipeline that is laid on the seabed or below it inside a trench
• Submarine Products – Former British diving equipment manufacturer and distributor.
• Submarine rescue – Rescue of personnel from a disabled submarine
• Submarine Rescue Diving Recompression System – Remotely operated vehicle for rescue of personnel from sunken submarines
• Submarine rescue ship – Support ship for submarine rescue and deep-sea salvage operations
• Submersible pressure gauge – Pressure measuring instrument for underwater service
• Subskimmer – Submersible diver propulsion vehicle with inflatable buoyancy for surface use
• Subsurface (software) – Open source software for logging and planning scuba dives
• Suit squeeze – Barotrauma caused by the folds of a diving suit piching the skin
• Superoxide scrubber – Breathing gas scrubber that adds oxygen and removes carbon dioxide
• Supersaturation – State of a solution that contains more solute than can be dissolved at equilibrium
• Supervised diver – Minimum requirements for a recreational diver to dive in open water under direct supervision
• Supervised Diver – Minimum requirements for a recreational diver to dive in open water under direct supervision
• Supply lock – Small airlock to transfer supplies between areas of different pressure
• Support diver – Recreational diving equivalent of a stand-by diver
• Surface blackout – Loss of consciuosness immediately following surfacing of a freediver
• Surface decompression – Decompression in a diving chamber out of the water
• Surface decompression on oxygen – Staged decompression obligation done in a decompression chamber
• Surface interval – The time spent out of the water between dives
• Surface marker buoy – Buoy towed by a scuba diver to indicate the diver's position
• Surface oriented diving – Underwater diving in which the diver starts and finishes at surface pressure
• Surface stand-by diver procedures – Functions of member of a professional diving team
• Surface-supplied air diver – Person competent to dive with surface-supplied breathing apparatus
• Surface-supplied breathing apparatus – Equipment to supply a diver with breathing gas at ambient pressure from the surface
• Surface-supplied diver, also known as Surface supplied diver – Underwater diver breathing gas supplied from the surface
• Surface-supplied diving – Underwater diving breathing gas supplied from the surface
• Surface-supplied diving emergency procedure, also known as Surface-supplied diving emergency skill – Standard procedures for use in common surface-supplied diving emergencies
• Surface-supplied diving equipment – Equipment used specifically for surface supplied diving
• Surface-supplied diving procedures – Procedures used in the safe operation and use of surface-supplied diving equipment
• Surface-supplied diving skills, also known as Surface supplied diving skills – Skills and procedures required for the safe operation and use of surface-supplied diving equipment
• Surface-supplied gas management – Provision of breathing gas do surface-supplied divers
• Surface-supplied mixed gas diver – Person competent to dive with mixed gas from a closed bell
• Surface tension – Tendency of a liquid surface to shrink to reduce surface area
• Surface water searches – Procedures to find objects or persons lost at the surface of a body of water
• Surface tension – Tendency of a liquid surface to shrink to reduce surface area
• Surfactant – Substance that lowers the surface tension between a liquid and another material
• Surfer's ear – Common name for an abnormal bone growth within the external ear canal
• Surge (wave action) – The component of wave motion close to and parallel with the bottom
• Sustained load cracking – Metallurgical failure mode of cracking under a prolonged static load
• Suunto – Finnish manufacturer of compasses, dive computers and sports watches

Sw

• Swedish Armed Forces Diving and Naval Medicine Centre – Diver education and training; diving medicine research and development
• Swimfin – Finlike accessories worn on the feet, used for swimming, snorkeling and diving propulsion
• Swimming – Self propulsion of a person through water
• Swimming-induced pulmonary edema – Body fluid in the lungs while swimming.
• Swim-through – Short underwater tunnel with adequate clearance and obvious exit

Sy

• Systemic circulation – Portion of the cardiovascular system which transports oxygenated blood away from the heart
• Systems technician – Person related to diving systems

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• T1200 Trenching Unit – Remotely operated seabed trenching unit

Ta

• Tactical Divers Group – Special operations force of the Argentine Navy.
• Taifib – Indonesian amphibious reconnaissance unit
• Talus cave – Cave formed by rockfall debris
• Tank weight – Ballast weight attached to a scuba cylinder
• Taravana – Decompression sickness after breath-hold diving
• Task loading – Relationship between operator capacity and the accumulated activities that must be done

Te

• Teaching method – Principles and methods used by teachers to enable student learning
• Team redundancy – System for sharing backup equipment and skill diversity
• Technical diver training – Processes to develop skills and knowledge to dive safely for technical diving
• Technical diving – Extended scope recreational diving
• Technical Diving International (TDI) – Technical diver training and certification agency
• Technical Extended Range (TXR) – Recreational scuba and freediving training and certification agency
• Tektite habitat – Undersea laboratory and experimental habitat
• Temperate reef – Hard benthic substrate and ecosystems in cool to cold water in middle latitudes
• Temperature – Physical quantity of hot and cold
• Tension-leg platform – Type of offshore platform used in production of oil or gas
• Tesei, Teseo – Italian naval officer and pioneering military diver
• Test of pressure – Diagnostic technique to identify decompression sickness
• Testing and inspection of diving cylinders – Periodical inspection and testing to revalidate fitness for service
• Texas Finswimming Association – Official governing body for competitive and recreational finswimming in Texas

Th

• Thalmann algorithm – Mathematical model for diver decompression
• Thalmann, Edward D. – American hyperbaric medicine specialist and decompression researcher
• Tham Luang cave rescue – 2018 international rescue in Thailand
• The Darkness Beckons – History of UK cave diving by Martyn Farr
• The Last Dive – Non-fiction book by Bernie Chowdhury about a double wreck diving fatality
• Therapeutic decompression tables – Planned hyperbaric exposure profiles to treat decompression sickness
• Therapeutic recompression – Recompression to reduce symptoms of decompression illness
• Thermal balance of the underwater diver, also known as Thermal balance of divers – Conditions in which the temperature of a diver can remain stable
• Thermal balance of the saturation diver – Maintaining body heat in the saturation diver
• Thermal undersuit – Insulating undergarments for use with a dry suit
• Thermocline – Thermal layer in a body of water
• Thermodynamic decompression model – Early model in which decompression is controlled by volume of gas bubbles forming in tissues
• Thermodynamic model of decompression – Early model in which decompression is controlled by volume of gas bubbles forming in tissues
• Theseus (AUV) – Large autonomous underwater vehicle for laying fibre-optic cable
• The Shaft (sinkhole) – Karst feature in South Australia
• The Silent World – 1956 French documentary film co-directed by Jacques Cousteau and Louis Malle
• The Silent World: A Story of Undersea Discovery and Adventure – Book by Jacques-Yves Cousteau and Frédéric Dumas
• Three bolt equipment – Standard diving dress using three bolts to clamp the helmet bonnet to the corselet
• Three Star Diver – Fully trained and experienced recreational dive
• Through-water communications – Wireless diver voice communications equipment

Ti

• Tidal race – Fast-moving tidal flow passing through a constriction, forming waves, eddies and strong currents
• Tide – Rise and fall of the sea level under astronomical gravitational influences
• Tillman, Albert – American educator and underwater diver
• Timeline of diving technology, also known as Timeline of underwater diving technology – Chronological list of notable events in the history of underwater diving equipment
• Time to surface (TTS) – Estimated ascent and decompression time from a given point on a dive profile
• Tissue (biology) – Group of similar cells performing a specific function
• Tissue gas loading – Amount of gas dissolved in a tissue
• Tissue slab decompression model – A diffusion limited series tissue decompression model
• Tissue slab diffusion model – Diffusion limited series tissue decompression model

To

• Torricellian chamber – Air space in a cave chamber with pressure below atmospheric
• {{annotated link|Total Nitrogen Time}|abbreviation=TNT}
• Touboulic, Pierre-Marie – French engineer, inventor and writer
• Tourism on the Great Barrier Reef – Service industry in Australia involving recreational diving
• Towboard – Underwater survey equipment used to tow a diver

Tr

• Training dive sites – Underwater environments selected as suitable for diving skills training
• Transfer under pressure – Moving between pressurised vessels without decompression
• Transfer under pressure diving – Surface oriented diving from a closed bell and a chamber for decompression
• Travel gas – Gas breathed during the descent part of a dive
• Tremie – Equipment for underwater concrete placement
• Trial diving – Introductory scuba diving experience
• Tribonucleation – Mechanism for creation of microbubbles
• Trieste II – US Navy's second bathyscaphe
• Triger, Jacques – French geologist who invented the pressurised caisson (1801–1867)
• Trim weights (diving) – Diving weights distributed primarily to improve trim
• Trimix blender – Person competent to blend trimix breathing gases for scuba diving
• Trimix (breathing gas) – Breathing gas consisting of oxygen, helium and nitrogen
• Trimix Scuba Association (TSA) – Recreational technical scuba training and certification agency
• Trongle – Device used on submarines to help swimmers to locate a submerged submarine
• Tropical coral reef – Reefs built by warm-water coral species
• Try-dive – Supervised diving experience to try out diving or equipment

Tu

• Tubesuit – Active heat transfer undergarment
• Turbidity – Cloudiness of a fluid
• Turkish Underwater Sports Federation (TSSF), also known as Türkiye Sualtı Sporları Federasyonu – Turkish national governing body for underwater sport and lifesaving

Tw

• Twin bladder buoyancy compensator – Scuba diving buoyancy compensator with redundant air bag
• Twin-hose regulator – Scuba regulator with two ambient pressure hoses to the mouthpiece

Ty

• Type 1 wet bell – Open diving bell without onboard gas panel
• Type 2 wet bell – Diving bell with onboard gas panel
• Type 904 dive tender – Chinese class of naval diving support vessel

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• Umbilical cable – A cable and/or hose bundle which supplies required consumables to a remote user
• Umbilical management, also known as Umbilical management (diving) – Safe handling of the diver's umbilical
• Umbilical tending – Management of the supply end of the diver's umbilical

Un

• Uncontrolled buoyant ascent – Type of diving accident
• Uncontrolled decompression – Unplanned drop in the pressure of a sealed system
• Undersea and Hyperbaric Medical Society (UHMS) – US based organisation for research and education in hyperbaric physiology and medicine.
• Undersea Medical Society – Original name of Undersea and Hyperbaric Medical Society
• The Undersea World of Jacques Cousteau – Documentary television series about underwater marine life
• Undersuit – Clothing worn under a protective suit
• Undertow (water waves) – Return flow below (nearshore) water waves.
• Underwater – Aquatic or submarine environment
• Underwater acoustic communication – Wireless technique of sending and receiving messages through water
• Underwater acoustic positioning system – System for tracking and navigation of underwater vehicles or divers using acoustic signals
• Underwater acoustics – Study of the propagation of sound in water
• Underwater archaeology – Archaeological techniques practiced at underwater sites
• Underwater archaeology techniques – Methodology used in underwater archaeology
• Underwater Bike Race – Annual charity event in North Carolina
• Underwater blackout syndrome – Loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive
• Underwater breathing apparatus – Equipment which provides breathing gas to an underwater diver
• Underwater camera housing – Water and pressure resistant container for camera
• Underwater citizen science – The organised collection of underwater data for scientific purposes by volunteers
• Underwater computer vision – Subfield of computer vision
• Underwater concrete placement – Positioning freshly mixed concrete at an underwater location where it is to set
• Underwater construction – Industrial construction in an underwater environment
• Underwater Construction Teams – Navy construction battalion underwater construction units
• Underwater cutting and welding – Metalworking techniques used by underwater divers
• Underwater cycling – Stunt in which a bicycle is ridden under water
• Underwater Defence (Turkish Armed Forces) – Special operations unit of the Turkish Navy
• Underwater demolition – The deliberate destruction or neutralization of man-made or natural underwater obstacles
• Underwater Demolition Command – Special warfare unit of the Greek Navy
• Underwater Demolition Team – US Navy special operations group
• Underwater diver – Person who descends below the surface to interact with the environment
• Underwater diver training – Processes by which people develop the skills and knowledge to dive safely underwater
• Underwater divers – People who participate in underwater diving
• Underwater diving – Descending below the surface of the water to interact with the environment
• Underwater diving emergencies – Situations that endanger the life or health of a diver
• Underwater diving environment – The underwater environment to which a diver may be exposed
• Underwater diving equipment – Equipment used to facilitate underwater diving
• Underwater diving hazards – Agents and situations that pose a threat to the underwater diver
• Underwater diving on Guam – Recreational diving tourism destination
• Underwater diving terminology, also known as Underwater diving terms – Definitions of technical terms, jargon, diver slang and acronyms used in underwater diving
• Underwater domain awareness – Underwater component of maritime domain awareness
• Underwater environment – Aquatic or submarine environment
• Underwater exploration – Investigating or traveling around underwater for the purpose of discovery
• Underwater Explorers Club – British diving club, now defunct
• Underwater firearm – Firearms that can be effectively fired underwater
• Underwater football – Underwater team sport using snorkeling equipment and an American football
• Underwater glider – Type of autonomous underwater vehicle
• Underwater habitat – Human habitable underwater enclosure filled with breathable gas
• Underwater hockey – Underwater sport of pushing a puck into the opposing goal
• Underwater hockey in Australia – History and organisation of the sport in Australia
• Underwater hockey in Turkey – History and organisation of the sport in Turkey
• Underwater Hockey Wales – Welsh underwater hockey association
• Underwater Hockey World Championships – International event for the sport of Underwater Hockey
• Underwater ice hockey – Freediving variant of ice hockey played upside-down under the ice
• Underwater inspection – Examination of underwater structure and equipment to assess condition
• Underwater mining – Any mining activity done underwater
• Underwater navigator – Diver competent to navigate in open water by compass or pilotage
• Underwater Offence (Turkish Armed Forces) – Special operations Forces of the Turkish Navy
• Underwater orienteering – Underwater compass navigation and speed competition on scuba.
• Underwater Orienteering World Championships – International sporting competition
• Underwater photography – Genre of photography
• Underwater photography (sport) – Competitive underwater digital photography on scuba
• Underwater Photography World Championships – International event for the sport of underwater photography
• Underwater pilotage – Diver navigation using natural features
• Underwater Port Security System – Defense against hostile swimmer incursions
• Underwater rescue – Rescue of persons trapped underwater
• Underwater rifle – A man-portable, long-barreled firearm which can be fired effectively underwater
• Underwater rugby – Underwater team sport
• Underwater rugby in Australia – Water sport
• Underwater rugby in Colombia – Regional sporting activity
• Underwater rugby in the United States – Class of breathhold watersport
• Underwater Rugby World Championships – International sporting competition
• Underwater search and recovery – Locating and recovering underwater objects
• Underwater searches – Techniques for finding underwater targets
• Underwater Society of America (USOA) – American national representative organization for underwater sport.
• Underwater sports – Competitive underwater recreational activities
• Underwater sports at the 2013 Bolivarian Games – Finswimming, free-diving and spearfishing events at San Lorenzo island
• Underwater survey – Inspection or measurement in or of an underwater environment
• Underwater surveying – Inspection or measurement in or of an underwater environment
• Underwater target shooting – Breathhold underwater sport of target shooting with a speargun in a swimming pool.
• Underwater tending point – Place where the diver's umbilical is restrained or controlled
• Underwater vehicle – Vehicle intended to operate underwater
• Underwater videographer – Diver trained in the use of video recorders underwater
• Underwater videography – Electronic underwater photography concerned with capturing moving images
• Underwater visibility – Distance at which an object can be seen through water
• Underwater vision – The ability to see objects underwater
• Underwater warfare – One of the three operational areas of naval warfare
• Underwater work – Work done underwater during diving operations
• Underwater wrestling (disambiguation)
• Under way – State of a vessel
• United Diving Instructors (UDI) – Recreational diver training and certification agency
• United States Marine Corps Combatant Diver Course – Military diver training for the US Marines
• United States Marine Corps Force Reconnaissance – USMC deep reconnaissance unit
• United States Marine Corps Reconnaissance Battalions – Reconnaissance assets of Marine Air-Ground Task Force
• United States military divers – Underwater divers employed by the US armed forces
• United States Navy Experimental Diving Unit (NEDU) – The primary source of diving and hyperbaric operational guidance for the US Navy
• United States Navy SEALs – U.S. Navy special operations force
• United States Navy SEAL selection and training
• Universal Referral Program – System to complete recreational scuba training with another instructor
• Unmanned diving – Underwater diving by autonomous or remotely operated vehicles

Up

• Upstream scuba manifold – Manifold connecting cylinders upstream of the valve
• Upstream valve – Valve which is opened against the pressure differential
• Upwelling – Replacement by deep water moving upwards of surface water driven offshore by wind
• URF (Swedish Navy) – Swedish submarine rescue vehicle

Us

• U.S. Divers – Manufacterer of scuba diving equipment
• User respiratory interface – Facepiece of a breathing apparatus
• US Navy decompression models and tables
• U.S. Navy Diving Manual (USNDM), also known as United States Navy Diving Manual, or US Navy Diving Manual – Training and operations handbook
• U.S. Navy Experimental Diving Unit (USNEDU, NEDU) – The primary source of diving and hyperbaric operational guidance for the US Navy

Uw

• Uwatec – Manufacturer of diving equipment

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• Valsalva device – Pad in a helmet or mask used to block the nose to assist in ear clearing
• Valsalva maneuver – Technique for equalising pressure in the middle ears
• Valve cracking pressure – Pressure difference needed to open a valve
• Variable weight apnea – Deep freediving using a weighted sled for descent, pulling along the depth rope for ascent
• Variable weight apnea without fins – Competitive freediving discipline
• Variable-buoyancy pressure vessel – Type of rigid buoyancy control device
• Variable-buoyancy propulsion – Use of a buoyancy engine to provide propulsion
• Variable density buoyancy compensator – Equipment for controlling the buoyancy of a diver or submersible
• Variable volume buoyancy compensator – Equipment for controlling the buoyancy of a diver
• Varying Permeability Model – Decompression model and algorithm based on bubble physics
• Vasa (ship) – 17th-century Swedish warship

Ve

• Venting the rebreather loop, also known as venting the loop – Intentionally releasing gas from the breathing circuit}
• Venture One diving accident – Saturation diving fatality in the North Sea in 1977
• Vertical Blue – Freediving competition held annually in The Bahamas at Dean's Blue Hole
• Vertigo – Type of dizziness where a person has the sensation of moving or surrounding objects moving

Vi

• VideoRay UROVs – Series of inspection class remotely operated underwater vehicles
• Viewport (diving) – Transparent part of a helmet, submersible, or diving chamber
• Vintage scuba – Early model scuba equipment and the ongoing activity of diving with it

Vo

• Volumetric gas fraction – Dimensionless quantity
• Voyage to the Edge of the World – 1976 French nature documentary
• VVAL 18 – Mathematical model for diver decompression

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Wa

• Waage Drill II diving accident – Fatal saturation diving accident in the North Sea in 1975
• Wall diving – Underwater diving alongside a near vertical face
• WASP atmospheric diving system – Type of armoured diving suit without leg articulation
• Water polo cap – Headgear used in water polo and a number of underwater sports
• Waterproof digital camera – Digital still camera with inherently waterproof casing
• Water safety – Human safety in the vicinity of bodies of water
• Water separator – Device to remove condensed water from compressed air
• Water surface searches – Procedures to find objects or persons lost at the surface of a body of water
• Watertight zipper – Zipper closure which prevents water leakage when closed
• Waves and shallow water – Effect of shallow water on a surface gravity wave
• Wave shoaling – Effect by which surface waves entering shallower water change in wave height

We

• Weight – Force on a mass due to gravity
• Weight belt – A ballasted waist belt worn by a diver
• Weighted boots (diving) – Shoes with heavy metal soles used as part of a diver's weights
• Weighted shoes (diving) – Shoes with heavy metal soles used as part of a diver's weights
• Weight harness – Webbing system to support diver ballast weights
• Weight pocket – Container on diving equipment to hold ballast
• Welfreighter – British midget submarine of the Second World War
• Wells, John Morgan – Physiologist, aquanaut and researcher (1940–2017)
• Welman submarine – Second World War one-man British midget submarine
• Welsh Association of Sub Aqua Clubs – Welsh national governing body for underwater sport and recreational diving
• Wet Nellie – 1976 car-shaped submarine from the James Bond film The Spy Who Loved Me
• Wet sub – Ambient pressure diver propulsion vehicle
• Wetsuit – Garment for thermal insulation from water
• Wetsuit boots – Foam neoprene footwear for watersports
• Wetsuit buoyancy – Upwards forece generated by the volume of an immersed wetsuit
• Wetsuit buoyancy loss – Temporary reduction of volume due to compression at depth

Wi

• Wildrake diving accident – Fatal offshore diving accident in Scotland, 1979
• Willful violation – Deliberate or negligent violation of workplace rules and policies
• Wind generated current – Flow in a body of water generated by wind friction on its surface
• Wind wave – Surface waves generated by wind on open water

Wo

• Women Divers Hall of Fame – International honor society.
• Woodville Karst Plain Project (WKPP) – Project and organization to map the underwater cave systems of the Woodville Karst Plain
• Work of breathing (WOB) – Energy expended to inhale and exhale a breathing gas
• Working diver – The person who is intended to do the underwater work during a professional dive
• Workplace health surveillance – Ongoing surveillance of workers' health
• World Recreational Scuba Training Council (WRSTC) – Scuba diving standards body
• World Without Sun – 1964 film by Jacques Cousteau

Wr

• Wreck Alley – Area off California with several wrecks sunk as artificial reefs
• Wreck diving – Recreational diving on wrecks

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• YMCA SCUBA Program – Defunct recreational diver training and certification agency
• Yoke connector – Clamp to connect scuba regulator to cylinder valve

See also