Not too Difficult
It shouldn't be too difficult. My '81 30' has the water heater installed in the bottom of the port lazerette. The water hose that goes from your engine to the exhaust fitting at the muffler is re-routed to run through the water heater first, then to the exhaust/muffler. The water heater is also hooked up to the shore power circuit so you have hot water at the dock. Don't forget that you still need a vacuum break "U" fitting in the hose circuit.