I'm inclined to agree with your mechanic friend.
When my old, old, old Columbia had a raw water cooled Atomic 4 (one of the three that it did) I had to bypass the water heater coil from the engine because the salt water ate the aluminum circuit.
When I repowered with diesel, the Universal's dimensions required repositioning the new water heater that is served by the coolant system of the engine.
It obviously works best when the hot water tank is full ( providing, if you will, a larger heat exchanger in addition to the engine's three inch mounted exchanger.) However, that requires leaving the demand water pump on to keep the water heater full of water when under way. Sometimes the fresh water supply in the main tank is less than enough to keep the water heater full and consequently, it doesn't cool the engine circuit as much.I also don't like to leave the fresh water system "charged" with the pump when underway, since a leak or broken hose will drain the tanks into the bilge.
The mechanic who installed the 25XP was adamant that plumbing in the water heater required a extra coolant tank and overflow recovery system above the level of the water heater to increase the coolants efficiency.
It generally runs 165 degrees F if everything is fine. As I noted before, restriction in the raw water inlet and failure to recover the overflow current would increase the temperature to 190. Not catastrophic, but indicative of a problem.
Check the engine to water heater circuit and make sure the tank coil isn't the problem.If there's not enough water in the tank, the air will heat much faster than water.