You will definitely need a tow valve in the unfortunate case of breaking down and needing your jet ski towed back. Without a tow valve installed you cannot go faster than 6 knots back to the ramp. Going faster than 6 knots causes water pressure to be sucked up through the intake pump into the exhaust, working its way back up to the engine exhaust valves and into your motors cylinder, filling the inside of your engine with water. No not a very good time at all, specially for your bank account.
Below is a photo of a tow tap fitted Yamaha FX SVHO. They cost around $30 or so to buy and take anywhere up to an hour to install, so it is not a bank breaker to have one. You turn the tap lever like you would with your kitchen sink to the off position to block water going up your main line into the engine.
If you ever have to tow a jetski behind a boat ... at only 6 knots.... what a nightmare!!