You cannot tow a ski behind a boat!!!!! If you do, water will get into the exhaust, and with the force of pulling, spill back over the exhaust into your cylinders, filling your engine/crankcase with water. DO NOT TOW YOUR JETSKI UNLESS ITS AN EMERGENCY!!!!
and only then you should go very very slow.
It will go into the intake, and flow backwards thru the system until it reaches the cylinders. When I say exhaust, I mean the exhaust system. A shutoff valve is only good as a precaution for flushing, when towing you have ALOT of water moving INTO the system.