Clementise..
Firstly running the engine to produce hot water only happens when you have a hot water heater that is connected to your engine. Some boats have them some do not. Those that do not and are used to cruise with often have the new owner change out the water heater.
If you have a water heater it should be located somewhere in or near the engine compartment.
It may look like one of these:
You might try sitting at the dock, plugged into power, and turn the switch labeled "Water Heater" on. This would be an AC power switch. Give it an hour and see if the water is hot.