Shouldn't be any harm whatever. If you think about the way the heater circuit works in your car (at least those without climate control), no water flows through the heat exchanger in the car when the heater is turned off. I once had a heater hose in my '73 Toyota blow while driving on a freeway. Lots of steam. I made a loop out of the good piece of hose bypassing the heat exchanger and drove 100 miles to home. No problem.

The water moving through the cooling system -- and ultimately through the HW heater -- is roughly 180+ degrees. All of the areas that this water is moving are well insulated and / or designed to handle this heated water.

In the hot water heater, the fresh water in the HW tank acts to dissipate the heated water moving through the HW tank from the heat exchanger. I would be reluctant to run heated water from the engine heat exchanger through an empty tank -- there may even be something in the warranty info that says to make sure you have water in the tank before applying heat.

Another option would be to shut off the flow of water from the engine's heat exchanger to the HW heater all together. (I have a gate valve in area neat heat exchanger especially for this purpose.) This will insure the flow of cooling water through the heat exchanger and keeping the engine happy; without running any heated water through an empty hot water heater, potentially making the HW heater unhappy.

Rick Caauwe
Moon River #550
Brunswick, Ga

My thought is twofold:

1. If there is no fresh water in the heater, you had better pop the circuit breaker for the shore-power input to the heater. Running the electric element with no water will be very bad.

2. If you are only worried about the engine-coolant portion, I doubt that will be a problem. Once the engine coolant reaches 180F (or whatever your boat will reach), it isn't going to get any hotter unless you have an engine cooling problem. It should not matter that there is no fresh water to dissipate the heat, since once the fresh water reaches that temp, there is no more heat dissipation occurring anyway. It is 'steady state' at that particular temperature.

Just my opinions.

Why not just simplify it: remove the lines from the engine to the heater from above the fresh water coolant pump and below the thermostat and stick in a 3/8" hose.

Another simple approach, since assuming you are going to repair or replace your WH, just disconnect the in and out hoses on the WH and connect them together with a smaller connector and the existing cable clamps, until you repair or replace, then just re-attach them.
Sam

Good idea, Sam, other end of the same hoses.