These people inhabit an underwater city.
In the city there are pools that connect the dry habitat with the outside ocean, this of course mean that the habitat is unpressurized and therefore the internal atmospheric pressure is the same as the pressure at the bottom of the sea.
So my question is: how deep can the city be?
I do not care what they breath as long as the air mix can let them live as deep as possible without any major problem. I also know that they cannot simply go back to the surface; to do that they need to undergo decompression, but that's ok, they are supposed to live there!