[August]

The thing is you have to be able to take in an entire lungful of air in order to breath properly.

That becomes a problem at depth because gas has to be compressed to overcome water pressure. If I remember my SCUBA lessons right, every 33' down gas compresses by half it's volume. That means at say 100' feet every breath you take is the equivalent of 4 lungfuls of air and therefore 4 times the amount of oxygen molecules with each breath. Eventually if you go deep enough you can take in a toxic amount of oxygen just by breathing normally and that's using regular air.

Now if you're breathing in pure oxygen like this device creates then you don't have to go very deep before you get to that point that's why it has to be have some other gas mixed in with it.