Rising coolant in the header tank as the engine warms up and the coolant expands is normal, the rest is not.
I assume that your only source of how water is a calorifier - if not, please come back and give more info because valves might be involved.
Your talk of bleeding the "radiator/coolant system" suggest that either your boat is not typical or you do not understand how the radiator (central heating) system, hot water, and cooling system interact. Please tell us about any other sources of keeping the boar warm that you have.
On the basis of the little you have told us, I would suggest that the engine coil in the calorifier has an air lock and needs bleeding. Trace the pipes between the calorifier and engine, there should be two about 5/8" or so in dimeter. Make sure there are no upward bends in them that can trap air. Then loosen the upper calorifier coli connection and see if any air is vented.
Your domestic how water system might have a thermostatic mixing valve in it, usually close to the calorifier, and if this is maladjusted or stuck then that might limit the domestic hot water temperature. After an hour or so running, feel the hoses on the engine calorifier coil, both should feel hot. If they do not perhaps the engine cooling thermostat has jammed open, so what is the engine running temperature.