Water leaks....
It seems that this is not an uncommon problem. I saw several Seadoo's last year and several more this year, including my own.
When you remove the tuned pipe exhaust system, you'll notice there are these round knob looking pieces. From what I see of it, this is where the water jacket was formed inside the exhaust system. The water crosses the head as it comes in from the jet pump, then over to the tuned pipe where it circulates to preheat the water before entering the engine block.
The corrosion your going to find is a white powdery substance called aluminum oxide. When water is heated up, the minerals and particulates will cling to the aluminum surface. This isn't really to good, as it causes insulation and really reduces the amount of heat transter taking place.
What I did to my 97 model 787cc: I took off the tuned pipe for, at the time, one leak. Well, as I worked on that one leak, it turned out to be 4 or 5. I used a dremel tool with a burrr bit and started at the small hole the leak was at. Before I knew it, the hole was the size of my thumb. Then, moving the burr bit around, pressing and tapping here and there, found several more. Most of them on those little humps. When you clean them, clean them well down to bare aluminum and the surrounding surface.
After exposing all your holes, if any are excessive, you'll need a piece of aluminum screen. Then, go to your local auto parts house and buy the strongest JB weld they have. They have a marine application for using underwater, you don't need that one. You just need a strong bond, that says it will work with aluminum.
Then, cut and mix a small, golfball size piece on a board or old plate. You have to knead it together really well (it comes in a tube with both parts together, one part inside the other). Do the smaller ones first to get the hang of it, then, if you have to use a piece screen, put the screen over the hole, then apply a small amount onto the screen where it overlaps the bad area. When it takes up, holding the screen on it's own, then apply over the top of the screen. I built mine up, probably close to a half inch.
I let it dry overnight, then, I put the sander on my dremel and smoothed it out. I cleaned the entire tuned pipe and got a nice, glossy black spray paint and gave it a coat..............
My repair has lasted through three tanks of gas and some tremendous bouncing on the waves will going air borne. It seems to be working really well. :cheers: