Well if the PTO is spinning freely at least your not looking at a seized crank. A compression test would be the next thing to check, but I understand this is not an easy thing to do when the starter won't turn the motor over lol.
I would say your next best step would be to pull the top off the motor and look down into the cylinders and confirm that the pistons, rings, and cylinders are still in decent shape.
other than that the next thing you could do to troubleshoot the issue is to remove the starter and connect it to a battery to see if its actually functioning correctly. This isn't a great test because a lot of times the starter can "look" like its in working order, but actually not be.
The best thing you can do otherwise is to disassemble the starter. its likely that the brushes are beat and need to be replaced.
here is a picture of starter brushes. The red one is old and the black is new. *This is not the same starter that will be in your ski just the best I could do on short notice* The brushes in your ski are a little different shape, but you get the idea. it's just a piece of metal that wears down over time.
They are very easy to replace and only cost around $20. Even if this turns out not to be your problem it won't have been a waste of time. Once you replace the brushes the starter will once again kick like a mule.
Heres a
link to someone tearing apart a Seadoo starter. The biggest thing to worry about is to mark the housing so when you put it back together you know it's going together correctly. If you assemble it in reverse of the way it's supposed to be the starter will work, but it will spin the opposite direction.
Also for the cost of a new solenoid, it might be worth just purchasing one and throwing it in to see if that does anything for you. They are cheap and unlike the starter non-OEM is fine for the solenoid