Before all the "Arm-Chair Developers" chime in on how easy it would be, here are the real devs explaining why we don't have underwater and why no flying mounts.
What is it with these underwater content threads? Not like they pop up that often but I really do not get it. Is it some kind of bandwagon all MMO devs have jumped on?
I would love this. The excitement of finding secret passages are rare treasure underwater in Oblivion was great. I the time I played BDO by far my favourite aspect was building a ship and going diving