User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-4639789-20120211213425/@comment-5020152-20120727160839

Certain prehistoric fish could use their swim bladder as a primitive set of lungs. Plesioth may do the same thing.

As for Gobul, he's definitely an air breather. There's no way he can breathe in the muddy waters of the Flooded Forest. And of course, the fact that he's more closely related to Lagiacrus (a Leviathan like himself) than Plesioth (a Piscene Wyvern).