I'm Pretty Sure Aussie Submariners Have Already Done This
Check out this story about an Australian who proposes to live underwater for two whole weeks:
Godson will spend two weeks inside the nine cubic metre box on the bottom of a lake near Albury, on the Victoria and New South Wales state border.Actually, several thousand submariners have also carried out similar experiments through the years. To be fair, though, while I do suppose that Collins-class boats are a little bigger than 9 cubic meters, you've got to fit a lot more people into them.
Removing carbon dioxide from the cell's atmosphere and producing oxygen from algae will be the most difficult elements of the experiment, during which Godson will also undergo psychological and physical testing.
"We've got some gas monitors and back-up air supplies to make sure that the air quality stays at a high standard," he said...
... Godson is not the first to carry out such a feat, as similar experiments were carried out by NASA bio-engineers in the 1990s.