Seems like after one of the dev builds the Speedboat wasn't working as expected. Mainly, when you rocked the boat left and right in a constant motion it created this strange stance where it seems to lifting off from the water surface. I firmly believe this is caused by a tweak in the Speedboat's water friction at dev build 107150 (01-07-2013).
I've demonstrated this in two YouTube videos. One being from the stable beta version 0.70 and one being the dev build 0.71.107360.