I think most people are kind of like "really?" when they go to Mist then just see their character can't handle going in anything past knee high water.

I don't think mounts that could cross water would be too big of a deal given the open world already supports hundreds of people and their mounts...and water is just water in that open world. I dunno about the boats and stuff, though. Right now bodies of large water are being used as invisible walls of sorts. I would imagine/hope there is some way to cross it eventually.