Unless you go really old, engines from jetskis are water cooled. The difference between a ski water cooled and say a sled or bike water cooled engine is the ski motor will not have a water pump. It uses water pressure from the jet pump to circulate water through the motor and exhaust. Bike water cooled engines will have a water pump built on the motor. Sleds will also have the clutch while the ski motors will have a screw on coupling similar to a "lovejoy" brand. No room for a clutch.
A friend will bail you out of jail, but a good friend will be sitting beside you saying "Man, that was fun!