I apologize if this had been mentioned.
a hand held infrared temperture sensor.
I find them extremely useful for diagnosing heat/cooling problems.
temp outside boat.
.....aim at raw water
hose as it exits engine
...note temp differential
Aim at Freshwater in to heat exchanger
and out of exchanger note temp differential
The raw water
out should be close to the difference between in and out of heat exchanger
BTW aim at your temp sensor and expansion tank....compare with reading on Temp Gauge of engine.