I had a '92 50hp Mariner that had the same problem with the impellers. I had to replace them every 1-2 years. A friend told me that they were engineered too small and once they became a little "stiff" they couldn't pump enough water through - came from a friend, not sure if it's legit or not.

I never had to replace the sensor. Wonder if it could be replaced with one that would let the water be a couple of degrees warmer cuz the water always felt fine coming out of the pee hole while the alarm was ringing so even the deaf could hear it - it never was even close to hot.

While I had that motor I think I replaced the impeller 3 times in 5 years. Not trying to scare you, but I think it's just the norm for those motors. They might call it "routine maintenance".