Heres a solution.
Work it like chloroplasts.
Any air an animal receives while under water will be from the dissolved air in the water (hence why tropical fish tanks have pumps among a few other reasons...)
Anyway, I'm not a complete expert on all the ways they do this, obviously one way is via gills, so more gills would result in more oxygenation.
I'd let you guys sort out the balancing, I presume perhaps the making of gills would cost, but after that its free? Anyway, thats how they get oxygen, then they in reality either use it to respire or be bouyant or both.