In trying to start my ski 1 week after riding it for hours with no probs, my overheat warning light comes on immediately after I try to start it now. Doesn't appear to have any blockage in the jet intake, intake grate, or in the impeller. I did manage to get the sensor off, clean it, and reinstalled it. I was able to crank on it 4-5 times before the overheat warning came on again. Any ideas?

It's possible the end of your exhaust pipe is clogged (#58 - where it connects to the water box). Remove it to visually inspect it.