Heat Exchanger for water heater, Raw water cooled engine.

[hole seeker]

First Post. On my 06 2700 SE I have a raw water cooled 350 mag. Hot water heater works fine when hooked up to shore power. I'd like to have hot water while at anchor, boat is in saltwater and often at Catalina with no shore power. I don't want to run a 2000W portable generator for a half hour to get a hot shower on day 2. Has anyone else plumbed the hot water heater to run off engine heated water? I realize there is a heat exchanger in the hot water tank, in addition to the electric heating element. I should be able to tap into the exhaust heated water and plumb it through the water heater heat exchanger and out a through hull fitting. So long as I freshwater flush after every trip I should be ok? Any input is appreciated. Thanks!