I have a 1993 Seadoo SPX.
On Sunday while jet skiing once I heard a slight quick squealing noise. Sporadically it would not reach top speeds..
Later it did a quick squeal and then the impeller seized up. It acts as if something was stuck, I have had that happen before, a small stone or plastic garbage getting stuck which I was able to clear.
This time I don't see anything there. Any suggestions? Could it be some kind of bushing or something?

Sounds like it was slipping and is now locked up, you will have to remove the pump to check it, first go to the back of the ski and remove the steering linkage, then remove the 4 bolts holding on the nossel, remove the nossel, now you will have to remove the 4 bolts holding the pump into the hull once those are removed you can remove the entire pump and driveshaft as one unit.
you will have to pry a little or pull hard to get it to pull out of the engine, then you can see if the pump is locked up try and spin the driveshaft/impeller if it will not move it is locked up.