heat sensor on Mercury 50 hp motor

I have a 50 hp 3 cylinder mercury outboard that is about 10 years old. Last Fall I had a repair shop change the water pump impeller and winterize the engine. When I got it out this spring, the sensor in the motor started to squeal when the engine was started. The water was coming out of the discharge chute just fine and the oil level was fine as well. the repair man said I needed a new heat sensor module which ended up costing $145 for the part. I am curious as to why this didn't happen when they changed the water pump last fall or is this just a coincidence?

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".