Mercruiser overheatingcontinuous alarm

I recently purchased a 1988 Th

I recently purchased a 1988 Thompson Daytona 260. I am experiencing the port engine running hot and the alarm (screatching noise) seems to stay on even when the temp and oil gauges indicate good oil pressure and the temperature is still indication 145-160 degrees. The starboard engine seems to have more power. Former owner says both engines were rebuilt with less than 25hrs on them.
I was thinking water pump? or oil sending unit? Mechanic said the alarm is when the engine shuts off but the engine is running so that theory is incorrect. Any ideas?

The alarm system uses seperate

The alarm system uses seperate pressure and temp switches. If you disconnect them one at a time you will find the culprit. There have been issues with the temp switch. Check this first.