The Sea of Cortes in summer is hot, sometimes between 100 and 110 F during the day. The water temperature climbs up into the low 90's. Since the fresh water in the tanks of Apolima matches the temperature of the sea water, showering with water at ambient sea water temperature is pleasant from about June to early October. The rest of the year I get complaints about the ice water coming out of the shower head.

As a good conscientious husband I listen to my wife and tell her I will arrange for her to have pleasantly warm showers. When I put my mind to solving this problem I found that there are a number of options open to us.

Run the engine so that the engine coolant will heat the water in the HW tank via the heat exchanger coil installed in the tank. Unfortunately the idiot who last replaced the hot water heater bought one without a heat exchanger coil. The only way to have hot water on Apolima is to have 120VAC. It works fine when we are plugged into the marina but we are rarely resting at a dock.

The easiest option we now have when away from the dock is to run the engine to produce 12 volts, to charge the batteries, to run the inverter, to heat the water. The downside is that this results in increased wear and tear on the engine and electrical system, noise and needless fuel consumption. Needless to say it is an expensive way to provide a warm shower.
Of course we can replace the hot water heater with one that has a heat exchanger coil so that water is heated every time the engine is run. Free heat, so to speak, as long as the engine is running for other purposes. The $600 + initial cost plus installation seems to be a serious downside to us retired on pensions. And the present heater is not broken yet.

The sun shines in Baja Sur just about every day of the year so lack of heat is not usually a problem so I designed a solar solution. Beautiful; yes, I'll Go Green. Simple. A coil of hose in a shallow box with a Plexiglas top is warmed by the sun. The warm water in the hose is circulated by a small pump to the hot water heater and the cooler water back up to the coil in the sun.

David on the catamaran 'Puddy Tat' pointed out that there was free heat available from the refrigeration system. EUREKA. Currently our refrigeration system is cooled by salt water pumped through a heat exchanger. If I cooled it by circulating the water in the hot water heater instead of salt water I would be able to make hot water for showers as well as eliminate salt water corrosion of the refrigeration heat exchanger.

I choose the last option. It required some hoses and valves and some plumbing changes. Now we have hot water every day when we run the refrigeration. It works.