I had one on my Chris-Craft Constellation and it worked fine, but I changed it out for a regular tank-type water heater with an engine heat exchanger. That way, anytime a main engine was running I was making hot water.
I also could make hot water without running the generator, as the tankless heater I had was a 120 volt AC model.
The tankless water heaters don't run a lot, but when they do, they really use some juice! I think mine was rated at 20 amps with 120 VAC power. I think I used #10 wire and a 25 amp breaker. I know you could hear the generator load up a bit when the water heater came on... 8)
Hope this helps - see ya on the water!
