Although not an authoritative answer, here's my take. I have a plane that weighs about 6 lbs on floats. The floats are 34 inches long. Its powered by a Thunder Tiger Pro 46 with a 11x5 prop. The prop strikes the water, but will launch the plane. I've also used a 10x7 prop with great success off the water. Now assuming you're not trying to fly, either of these combinations would probably overpower your boat. I definitely would stay away from the 60 size. You might even consder going smaller with a 28 or 35 size. Less weight to push around.

Brad