clutches
Sell the motor and buy one made for a boat. There is no way to water cool this motor unless you are a very good machinist and it does need water cooling. The powerband and low RPM is not made for boats.
As far as clutches goes, there was one company that made some but I don't know if they are still in business. Be careful around a spinning prop and you don't need one, same as an airplane.
The Dumas Scarab is a tall boat so I don't think that it could get enough air in the first place. Sorry, just trying to keep it in the real World.