Measuring The Raw Water Pump
The impeller is cooled by the ocean or lake water that it's pumping. It's a good idea to get a baseline reading on this as well. If you develop and blockage of your intake or sea strainer the temp of the raw water pump can climb due to less water flow and thus less cooling.
It will normally read a little hotter than the ocean temp as measured on the outlet side and this is normal. This reading, even with a brand new impeller, should still be well bellow 10 degrees.
Measuring the Hot Water heater Supply Line
In this photo I am measuring the temp of the supply to the hot water heater.
Measuring the Return Line From the Water Heater
As you can see my hot water heater is not up to temperature and I have about a 40 degree differential temperature across the supply and return lines for my water heater.
The water heater acts like another heat exchanger and can throw off readings and overheat findings. Your supply and returns from the water heater should be within about 5 degrees of each other before taking your baseline assessment.
I often hear "My engine runs fine for about 30-40 minutes then begins to overheat?". This could just be a situation of a clogged HX, sea strainer or other raw water circuit problem getting masked by the cool water in the water heater. This cool water acts like and can mimic a heat exchanger. As the temp in the heater rises the engine can no longer dissipate heat into that water and it begins to over heat.
Don't forget to write all these numbers down with both your high and low rage readings so that when and if you have an over heating situation you can easily trace it by comparing these numbers to the new readings during an over heat..