User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-1011548-20120818163931/@comment-5020152-20120819212435

The Volcano is near the ocean, if I recall. And maybe that moisture is in fact some kind of resin that helps to keep water locked within the Ioprey's body.

As for your second question, I think that they're more intelligent than the other 'preys, and are being opportunistic by stealing food from those areas. (Humans, having no natural weapons, are much easier to steal from than, say, a Rathalos. I guess they've realized this.)