What stops you from that though? Even if you can't breathe underwater, your body is more adapted to swimming right? So you'd be able to stay afloat a lot longer as well.
[uh] It's like a hole.
Eel makes sense then. If you were a sea snake I wouldn't be able to say if you'd get gills or not. Turtles and sea snakes both need air to breathe, so they just surface for that normally, it's not the same as with 'mer'.
Eels have round gills that are pretty similar to spiracles, so maybe you'll get those though?
< SPEAKING.IN.TONGUES >
[uh] It's like a hole.
Eel makes sense then. If you were a sea snake I wouldn't be able to say if you'd get gills or not. Turtles and sea snakes both need air to breathe, so they just surface for that normally, it's not the same as with 'mer'.
Eels have round gills that are pretty similar to spiracles, so maybe you'll get those though?