So not really something that applies to diving for the majority of people who dive, but when you jump into technical diving things get a bit more involved. I was curious about how much air I breath so I did some searching and came up with this interesting scenario. Check it out, it is a fun read if nothing else.
https://www.physicsforums.com/threads/how-long-can-one-live-in-a-sealed-room.685796/
Come on in, the water is perfect