My XP has a nipple inside the engine bay, on the hose above the spark plugs. It has a rubber cap over it. If your ski has this kind of fitting, you will need to buy a special hose from Seadoo that has the garden hose fitting on one end, and a spring-loaded snap-on fitting on the other that fits over the nipple.
Seadoo manual says to use something to pinch that hose behind the nipple, to the rear (I use vice grips, set to just gently pinch the hose closed, not so hard as to damage the rubber hose), and use low water pressure so you don't force more water into the engine than the engine can use and flush out the exhaust. If you don't pinch it, the water will just run straight out the back of the ski without flowing through the engine. Some may go through the engine, but most will just flow out the back, where there is less distance and resistance.
Then follow Karl's instructions on flushing the engine.