I'll be doing some extended cruising next year in places remote enough
that shoreside shower opportunities may be few and far between, and
the water pretty cold in August, let alone October, so I am
considering an engine heated hot water system.

The dumb idea is: "How about just diverting some engine jacket water
straight to my cold water shower fixture." I'm in fresh water, and
anywhere I would want to use it would be pretty clean fresh water.
Even with a heater tank I would probably use lake water in the system.
Comments?

Assuming that really is a dumb idea, how about recommendations and
caveats for installing a hot water system? How big a tank do you need
to get a satisfactory shower? Does running the engine while showering
extend that capacity a lot?

Thanks,

Rick