The aquastat I mentioned isn't all that expensive and it does exactly what I want. There are also at least two other manufacturers making the same thing that might be a bit less money.
OK -- I don't know what they are asking for that part. Snap-disc single-value temperature control switches are about ten bucks.
What I am looking to do is turn on or off my Proheat heater based on if the water from my water heater is hot or not. I would also need a mixing valve or something to sure the water in the water heater doesn't get too hot if the heater runs a long time for space heating needs.
Maybe I am overcomplicating things. The Proheat will turn itself off if the coolant level coming back to the heater is too warm.
Your ProHeat will cycle on and off based on the hydronic fluid temperature -- the hottest your domestic water will get is that same temperature, but more realistically, there will be enough loss in the exchange and from the tank that the water will run 10-20° lower.
FWIW, our Webasto cycles off at about 165° (and that's probably how hot the hydronic fluid gets from main engine heat, too, after going through the exchanger). The thermostat on the electric element of our water heater (one of those same $10 snap-disc items) probably cuts out around 145°, yet the water is always hotter if we heat it electrically rather than hydronically.
I would just not worry about it. OTOH, if you have or intend to have small children aboard, you might install an anti-scald valve on your shower. We don't have one and have never had a problem. There are also "tempering valves" available that go on the output of the water heater, if you are still concerned about it.