First, I confess I have only quickly skimmed all the posts, so this may have come up. But why not one of the on-demand gas heaters? I confess I know little about using one on a boat, but I have seen a few installed. They don't take up a huge amount of space and just heat the water as you need and (I assume) as much water as you want. They are really gaining in populartiy in residental use and always struck me a great idea. Since you don't see them very often, maybe there is a big disadvantage I am not aware of?