User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-4639789-20120211213425/@comment-1011548-20120213182418

Plesioth is covered in scales, which tend to be fairly water-proof. It could be running back into the water because it prefers it then, or because it needs to wet it gills.

I'd be amazed if Lavasioth used gills, there's nowhere near enough oxygen in lava (as far as I'm aware). Plus its easy enough to just stick your head above the surface to breathe, and it probably needs to do that anyway to let it see prey.