I'll try and steer this thread back on track...
We are new to the cruiser field and have yet to use our boat for anything longer than 4-5 hours at once but I do know that my 6 gallon 120 V standard hot water heater (no engine heating) that is located in the rear most starboard engine bay has to go. I want that area for batteries. Realistically how much hot water do you need on a boat? I only see us using hot water to either wash some dishes or maybe our hair if we are on the hook. The lake will work as a big bath tub

Most of our time will be single overnight stays, longer trips will involve marina visits so we can shower there.
here's what I am going to install under the bathroom cupboard.
http://www.gsw-wh.com/pdfs/products/water/spacesaver/22451_1A.PDFThe way I see it, your using the engine to heat the water in those other types of heaters and if you travel in the morning by the time you go to use it in the evening it'll be luke warm anyways so your going to have to either start a genny or an engine anyways. Why complicate the matter with extra hoses and risk a leak?
You may want to consider something similar in a more accessible location.