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Hi and thank you for your question.
If it ran fine once around the lake, it is probably not a carb issue. This unfortunately sounds much more like a mechanical issue. I suggest you do a compression test on the engine first and foremost to see what the mechanical condition of the engine is. The losing power, not wanting to build RPMs then not wanting to start or turn over very much point directly to a mechanical issue.
Do you still have the oil injection on your ski or are you premixing? IF premixing, what ratio and what oil?
There is a starter relay, or solenoid as some people call them, inside the electrical box. I would suggest checking battery voltage first, then making sure all the connections for the cables are tight. Start with the battery connections, then check at the starter motor where the ground connects to one of the starter motor mounting bolts and of course the positive cable coming from the starter relay.
Can you grab the coupler by hand and turn the engine over when the spark plugs are out? If not, you don't have an electrical problem, you have a mechanical problem.
Have you been replacing the starters or is that just the ski history? If it you that is replacing them, I would try a different brand of starter. If you have used different brands (aftermarket) you need to know WHY they are failing. First is it an electrical problem or mechanical problem inside the starter? Have the brushes failed? Etc........ Without know why they failed, we can't make a decision as to what to do differently to prevent another failure. Also, how does this tie into the loosing power once around the lake? The only thing I can think of is if the bendix has seized and as the engine runs, it is spinning the starter motor and causing it to come apart because it isn't designed to turn the RPM's the engine can spin it while running. Normally, the bendix allows the engine to run and over-ride the starter by slipping (internally). If it gets corroded due to gaskets or seals failing, it locks up and is no longer able to freewheel while the engine is running. Think about the bendix as a one way clutch, it transmits power from the starter to the crank when the starter is engaged, and disconnects the crank from the starter once the engine starts.
The best thing to do at this point is to remove the bendix and inspect it and check its operation. If it doesn't freewheel in either direction, IE locked up both ways, it has failed and needs to be replaced. You will have to pull the front engine cover off and the flywheel to get the bendix off the engine. You can check its operation without pulling the flywheel however.