I have no idea if this is reasonable. I am just putting out there because I thought about it.
So we all know that some animals have lungs, some have gills, etc. I also learned last year that some amphibians can breath through their skin since it is so thin. I thought this was cool then it occurred to me...
Is it possible through surgery to move the blood vessels closer to the surface of the skin, enough to absorb oxygen from the water?