The night before the crew rotation, the NEEMO 21 aquanauts performed the extensive decompression protocol that allows them to return safely to the surface the next day. Once the body is saturated with nitrogen from the elevated pressure of living under the sea, it simply isn’t possible to return to the surface at will. This ~17 hour decompression is why Aquarius is considered an extreme environment, and a key reason it makes such a good spaceflight analog.
There was some required reading while they went through the O2 protocol... :) ????
NEEMO 21 has an international crew, similar to the International Space Station. Our aquanauts this mission are from United States, Germany, Ireland and South Africa.