Most seadoo's use 89 octane fuel or regular. If this is the case for yours than you don't really need any additive for regular everyday use. If you don't plan on riding it for a while than a stabilizer is recommended to keep the fuel fresh.
Karl
If you add more it won't hurt, because it is made to burn with the gas when you use it again. It prevents the additives from evaporating from the fuel.