If you were to throw a foreman in a 10 ft lake it would flip over and start floating tire side up. Im sure eventually all the air from the motor and diff's would get out and possibly it would go to the bottom.
But to answer your question they do not float. If any part of the foreman floats its the rear. The front kinda just goes up when you are accelerating through the water, as soon as you let off it goes back down.