Our float cabin is off the grid. You can live simply, or with lots of modern innovations. For hot water, many people have chosen tankless hot water heaters run by electricity (solar or generator) or propane. There are also wood stoves that generate hot water. You can make your own, or purchase one with that feature built in.
Wayne and I have chosen to adopt a simpler solution. We like to have hot water for cooking, washing dishes, cleaning, and winter tub baths (lake water is just fine in summer).
On bath day, four large pots on top of the wood stove will give us enough hot water for a bath in our regular size tub in our new bathroom. Plus the extra heat needed to get it boiling warms up the cabin for when we are done.
Add an equal amount of cold water and that's all you need for a good scrub and soak. Well, that's all you need for a shared bath that is. But it's double the fun to get clean with your good buddy. -- Margy