I've sailed on many boat with these systems and it is hard to complain
about hot water. Perhaps the second loop could be used to cycle water
through a solar hot water coil--the penalty would only be the
electricity to
circulate it.

Without an engine driven hot water system, you are stuck with AC coils
at the dock, or a huge electric penalty when underway.

The WhisperGen stirling engine offers hot water as a by product. I
don't
think that uses coolant.

As someone else mentioned. It would be wise not to drink hot water
using such a system. Heat up any drinking water separately. I don't
think diluted coolant will kill you if you only shower in it or wash
dishes
with it.

To avoid corrosion an isolation transformer is the best option. Be
sure not to
tie the grounds on both sides together. Incoming circuit ground goes
to
the transformer case, the other goes to your vessel ground, such that
no
DC current can pass from shore power to the vessel. With such a system
you can crank up all AC appliances and forget about corrosion.

Scout wrote:
How many here have a hot water heater that works with the engine's coolant system as a heat source?

It is a very thin line separating that toxic material from your drinking water supply. The closed engine coolant loop may reach 15 psi when hot, and could easily outmatch the force in the fresh water system, especially if you run dry or turn off your on-demand pump for any reason. A pin hole or a crack in the hw heat exchanger could have deadly results.

In my work as a boiler tech, we were never permitted to use toxic anti-freeze in boilers that produced domestic hot water (summer-winter boiler packages). I've replaced tens of those coils due to leakage!