Nine posts since 2000? This is not a person who asks for help often!
Another point to add to your mulling: Today I ran my old boat from one yacht club to another to do a repair. The Atomic 4 has the fresh water thermostat rated at 180F (good for the engine, but too hot for salt water, where it throws a deposit, and where the 'stat is rated 140F). Anyway, I was cruising at five knots for an hour, and I couldn't see the engine temp get above 145 F.
Then it struck me: The water temperature in Lake Ontario in May is about 9 C/ 45 F. That's pretty cold water going through the block. By July, it will be 21 C/70 F and I'll be getting readings of 175 F and presumably, the thermostat will be mostly open.
Could it be that the raw water cooling circuit is quite cold at the moment, meaning the engine coolant can't warm up unless that circuit is restricted?
The hot water circuit might be at issue, but it really depends on how it's plumbed. Mine not yet connected into the engine, which is coming out for inspection, but I am assuming that it is the raw water circuit that is heated by the coolant (in turn heated by engine heat) which in turn transfers heat to the potable water in the hot water tank. Otherwise, you'd run the coolant itself through the coils in the hot water tank, and that would be a little dangerous. A failure of the raw water circuit...post-heat exchanger...would be noticeable by the lack of water at the
exhaust, the change in engine note, and water in the bilge....nasty but fixable if noted before things post mixing elbow start melting.