I've heard about this problem from several sources - engines on brackets (like many of our Parkers) tend to take more wash when suddenly decelerating. As Dale wrote, water can wash back up the exhaust ports because when you're decelerating, there isn't sufficient exhaust pressure to keep the water from coming back up the exhaust and into the engine. If the water makes it past the valves, poof! you have piston, valve, ring, or rod damage (or all of the above). I heard about this happening to a local guy with a brand new motor - he got lucky and it was replaced under warranty (gotta love the 5-yr Honda warranty).