I built a boat in Snapshot 13w16a, placed it in water, and right-clicked to enter it. I got a java.lang.NullPointerException. I closed the client and opened it back up, tried to log in, and I got the same java.lang.NullPointerException.

I had to revert both the server and client back to 1.5 in order to log in again. When I did, both my boat and I were underwater, and I slowly rose to the surface.