OK, So I'll admit it. I took a bath with my boat and it floats. It stays about half a brick above the water line. However, If you get a little bit of water in the cabin area it rolls over very easily.
The speedboat however floats quite well. The rear end does stay considerably lower and swamps out the passenger area to waist deep for the minifig.
Most fun bath I've taken in some time.
~LCM