I stumbled across an enthusiast who has managed to create a working saturated submerged habitat for his pet hamsters, extraordinary because at 6 months in there they’re unofficial world record holders.
The habitat is fairly well designed… The air supply is from a surface mounted air compressor with 14hrs of battery backup in case of powercuts/mechanical failures etc. There is a terrarium (enclosed garden) intended to recycle air to and from the main ‘living’ area, a hamster wheel to keep them busy (preventing hamstronaut cabin fever), a dining area, air-lock to the surface, and plans for a moon pool where hamstrosubs can dock. And no it hasn’t leaked – the air pressure inside the habitat, even at 1m below the surface is enough to prevent water entering (There’s no need to watch the entire video, you’ll get the idea after about 5 seconds):
^^ Video shows more up to date habitat
After a few successful experiments he’s now sadly cut the funding to his undersea hamster exploration/colonisation programme and is working on a hamster space program (in a ruthless science funding decision similar to what happened in the 1960s with human ocean exploration vs space exploration).