This is common!....
This is a common occurance in the older model skis/boats. This tuned pipes purpose is to preheat the cooling water as it enters the engine, to minimize thermal shock to the engine block.
When this pipe was cast, it's a pipe inside a pipe. The leaks usually occur around the little welded knobs at the pipe. I had 4 or 5 leaks two seasons ago, patched it and have had no problems since. Even if you buy a used one, your libel to buy one with the same problem.
This thinning of the aluminum wall is caused by aluminum oxide, like galvanic corrosion. Whent the water is preheated, the elements and minerals in the water, leach out and cling to the metal surface. When you pull the ski from the water and these chemicals dry out, the corrosion sets in.
The fix is simple, it's just that you have to pay attention to detail. A dremel tool is the best thing I"ve found, to clean all the old soft spots out, to gain a nice clean surface of aluminum, before making the fix.
At the bottom of this page, I have a link to "snipes korner".....click on it and go to post number 5. There, you will find the step by step instruction in this fix. YOu have to remove the tuned pipe for the repairs......make sure you tap gently on the pipe, to find any further weak spots, so that you can repari them while its off. There is no real pressure on this system, outside a couple pounds...............follow the instructions and I promise, you'll find success in your DIY fix............if you have any problems...........PM me:cheers: