There was a thread about this in the mod section a while a go. Someone used that silvery noise blanket, and added it to the inside of the engine compartment. The final outcome was that the noise at idle was clipped... but at speed it didn't' make much of a difference. That's mostly because the volume levels at speed are from wind, and not the engine.
Wrapping the actual waterbox probably won't do much. But... if you add any sound proofing to the boat... make sure to post up some pics.