An aquanaut is any person who remains underwater, breathing at the ambient pressure for long enough for the concentration of the inert components of the breathing gas dissolved in the body tissues to reach equilibrium, in a state known as saturation. Usually this is done in an underwater habitat on the seafloor for a period equal to or greater than 24 continuous hours without returning to the surface. The term is often restricted to scientists and academics, though there were a group of military aquanauts during the SEALAB program. Commercial divers in similar circumstances are referred to as saturation divers. An aquanaut is distinct from a submariner, in that a submariner is confined to a moving underwater vehicle such as a submarine that holds the water pressure out. Aquanaut derives from the Latin word aqua ("water") plus the Greek nautes ("sailor"), by analogy to the similar construction "astronaut".

The first human aquanaut was Robert Sténuit, who spent 24 hours on board a tiny one-man cylinder at 200 feet (61 m) in September 1962 off Villefranche-sur-Mer on the French Riviera.^[1][2][3] Military aquanauts include Robert Sheats, author Robin Cook, and astronauts Scott Carpenter and Alan Shepard. Civilian aquanaut Berry L. Cannon died of carbon dioxide poisoning during the U.S. Navy's SEALAB III project.^[4][5][6] Scientific aquanauts include Sylvia Earle, Jonathan Helfgott, Joseph B. MacInnis,^[7] Dick Rutkowski, Phil Nuytten, and about 700 others, including the crew members (many of them astronauts) of NASA's NEEMO missions at the Aquarius underwater laboratory.

A unit of the Russian navy has developed an aquanaut program that has deployed divers more than 300 meters deep. An ocean vessel has been developed and is based in Vladivostok that is specialized for submarine and other deep sea rescue and that is equipped with a diving complex and a 120-seat deep sea diving craft.^[8]

A Nigerian ship's cook, Harrison Odjegba Okene, survived for 60 hours in a sunken tugboat, the Jascon-4. The vessel, which capsized on 26 May 2013 due to strong ocean swells, had been performing tension tow operations and stabilising an oil tanker at a Chevron platform in the Gulf of Guinea^[9] (in the Atlantic Ocean), about 32 km (20 mi) off the Nigerian coast.

After sinking, the boat came to rest, upside-down, on the sea floor at a depth of 30 m (98 ft). Eleven crew members perished. However, in total darkness, Okene felt his way into the engineer's office, where an air pocket space of about 1.2 m (3 ft 11 in), in height, contained just enough oxygen to keep him alive. There, he fabricated a platform from a mattress and other floating objects, keeping his upper body above the water to help reduce heat loss.^{[10][11][12][13]} According to Okene himself, prawns were nibbling on his feet and legs as he sat in the water, praying.

Three days after the accident, with authorities already en route to investigate, Okene was discovered by South African divers Nicolaas “Nico” van Heerden, Darryl Oosthuizen and Andre Erasmus, employed to investigate the scene and recover what they presumed would be twelve bodies. Upon entering the engineer’s office, van Heerden saw a human hand, belonging to Okene, which he assumed to be a corpse. As he pulled on Okene’s hand, van Heerden realised the hand was grasping onto his. Immediately, the diver surfaced within the small space to speak with and devise a survival plan with Okene. The rescuing divers determined the only option was to bring Okene the proper dive equipment, outfitting him with a diving helmet so he could breathe; after swimming out of the shipwreck, Okene was transferred into an enclosed diving bell and safely returned to the surface for decompression from saturation. Nonetheless, the stressful experience combined with the decompression transfer caused Okene to pass out; however, he was revived, and immediately taken to hospital by helicopter. Experts determined that given the amount of space Okene was living in underwater, he possibly had two or three days’ worth of oxygen remaining.^[9]

• Continental Shelf Station Two – Undersea research habitat in the Red Sea
• Jacques Cousteau – French naval officer, oceanographer, filmmaker and author
• World Without Sun – 1964 film by Jacques Cousteau
• La Chalupa Research Laboratory – Human habitable underwater enclosure filled with breathable gasPages displaying short descriptions of redirect targets
• Astronaut