User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-4639789-20120211213425/@comment-1011548-20120211215932

I'm pretty sure Gobul is an air-breather. It's classed as a Leviathan, which means despite appearances it's probably more closely related to reptiles than fish. Plus there's all the reasons you mentioned, as well as the fact he never really goes into deep water - sticking in shallowish water means he's never too far from the air.

Does Plesioth breathe air or use gills, though? There's an interesting question...