Because firstly, if you're underwater, you're going to be spending a significant time alone which sometimes I find very appealing. Secondly, I picture it in my head like having the underwater equivalent of the forest of nowhere all to yourself to go crazy on and rank up in form at a pretty good rate.
With that said, and with respects that this likely sounds like a request for a silver spoon, how easy/difficult is it to find the underwater areas for ranking?
As for finding a downside, I figure logically, gear might be tricky to get your mitts on unless you get a really good amphibious form to nurse you through killing a few mobs for gear...