Hopefully they don't get waterlogged underwater

Looks a little wet for the heroes, doesn't it? Yeah, they can explore underwater, too! As to how they are breathing, don't think too hard about it- this is the same game world that has giants like William walking about and not collapsing under their weight. Or they could be using the scuba gear key item, I suppose? Anyhow, if the party gets in too deep, they'll begin taking damage from the water pressure. And there are powerful monsters that lurk in the deep, as well!