Check for the most obvious problem first, an impeller that has broken off a few rubber vanes and is now partially blocking up the water pump
or heat exchanger
. Causing the engine to run hot.
We just repaired this issue on a friend's boat
yesterday. Repaired in on my own generator
last month. Same symptoms, same cure.
Impellers only last 18 months to two years at most despite what the manufacturers claim, so now we change ours each year at the beginning of the season.