The faster the engine rpm's the greater pump thru-put on the hot water pump. The discharge temperature of the gases through the muffler also varies.

By the time your engine exhaust hits (say 200 degrees) on a sailboat, you're already in a bunch of trouble.

Get a warning EARLIER, is my advice.

On a trip to 600+ NM passage to Maine I noticed that we had moisture in the bedding of a the forward cabin when I went to try-out the forward head. We had been motoring at 2800 RPM for a bunch of time. It was "hot" moisture.

I pulled up the bedding and access panel because I knew the hot water heater was under the head of the bunk (about even with the mast). The high temperature "pressure release" plug was letting off steam because the water in the hot water heater had gone above 212 degrees F. I thought I had a leaky plug. So I released the plug to exchange "cold water" into the tank. A few minutes later the cold water had become steam.

I went back to the helm, looked at the temperature guage and, sure enough, it was below "red line", but showed that it was relatively accurate about being at "boiling".

I had plenty of flow out the stern. It could have been many things. Apparent "Good" flow out the stern tube doesn't necesarily mean "full" flow. The first thing I had checked was the the impeller having lost a blade, but that wasn't it. I changed the impeller anyway underway that evening when I found my overheating.

When I tooled down to 2600 RPM, the temp dropped down.

I completed the run around the outside of Nantucket Shoals and up to Mt. Desert Island (SW Harbor, ME). Then we cruised for a week before I went to figure-out what the issue was. (The 50 degree water masked the issue unless you were motoring for a while at high rpms.)

To make a much too long story short, the problem was the the heat exchanger needed to be ultrasonically cleaned (NOT BOILED OR JUST SOAKED) in order to get the "hidden" deposits from effecting the efficiency of the exchanger.

The only symptom was the high RPM rise in temperature from NORMAL. Now, I can't help but look over to the temp guage periodically while motoring just to see if there is some indication of something being amiss.

There are often combination warnings that multiple sensors can provide you to help diagnosis issues. Having multiple bits of intelligency can sometime allow you to diagnosis what it happening before. In this case, experience and learning didn't come without some frustration, wasted time, and money.