I'm a little confused by your description, Bronco. Not sure if the reference to the calorifier and domestic hot water is a red herring or not. My boat produces sizzling hot water, although I've been told there is a thermostat somewhere, which I can't find.
Putting the domestic water system to one side, the first question is whether your raw water circuit is in order. Do you have a reasonable flow of water coming out of the exhaust? Is it warm but not hot when you put your hand in the stream? If the flow is very poor or non-existent, the first place to look is the impeller. If the impeller has lost a blade it won't be pumping very efficiently and the missing blade may be partially blocking the heat exchanger.
If the impeller is good, or if you have to go hunting the broken bits, the next stage is to take off the elbow at the front end of the heat exchanger. Poke something (wire coathanger, maybe) down each of the tubes to make sure they are clear. If they're not, you'll probably have to dismantle the whole heat exchanger to clean it up.
If you haven't found a problem yet, inspect the exhaust all the way back from the elbow to the outlet on the hull. I can't imagine what problem might occur there, but it's best to check.
If the raw water passes all the tests, check the coolant circuit. I've never touched this bit, but the obvious things would seem to be
Is there enough coolant?
Is the thermostat working properly?
Is the pump doing what a pump should do?
