The water surface is on the zero level, the game engine itself supports not only the movable objects, but even flying ones, so it should not be a big problem to create some vessels although there may be some difficulies with moving characters along the deck on a moving vessel. And no, there is noting flyable planned. =)
The thing is that sea vessels are far from real importance for dev team. There is no point to start assembling ferrari while you don't even have a dirtroad. So be patient and enjoy what we have at this point.
Seriously, nobody will change the dev plan before they create and implement all the features scheduled for the release. And that will not happen before the release
So it is quite logical that they plan all further changes for the post-release development.