Short answers, pulled from the story.
An aquanaut is a person who breathes at the surrounding water pressure long enough for the inert gases in their breathing mixture to fully dissolve into their tissues, a state called saturation. This is usually done inside a seafloor habitat for 24 continuous hours or longer without surfacing.
Robert Sténuit is generally regarded as the first aquanaut. He spent 24 hours inside a one-man cylinder 200 feet underwater off Villefranche-sur-Mer on the French Riviera in September 1962.
An aquanaut breathes at the surrounding water pressure, while a submariner stays dry inside a moving vessel such as a submarine that keeps the ocean's pressure out.
Berry L. Cannon, a civilian aquanaut, died in 1969 of carbon dioxide poisoning during the U.S. Navy's SEALAB III project.
Harrison Odjegba Okene, a Nigerian ship's cook, survived 60 hours in an air pocket inside the sunken tugboat Jascon-4 after it capsized on the 26th of May 2013. He was found three days later by three South African divers and decompressed from saturation over about two and a half days before reaching the surface.
A unit of the Russian Navy runs an aquanaut program that has deployed divers to depths beyond 300 meters. It operates a Vladivostok based vessel equipped with a diving complex and a deep sea diving craft that seats 120 people.