I have only very recently started looking into rocket stoves and the possible uses for them. I think there may be another way to be completely off grid and still have a nice warm bath when you want it. I saw this website several years ago and I think it would be possible to incorporate the rocket stove exhaust, or perhaps the actual stove, with the dutch tub.
I'm just thinking that if you could run the exhaust tube straight up the middle of the coil you would be able to have a nice hot bath every day. I also found the exhaust system on the wooden one to be of interest. I think it would assist in heating the water much more effectively. they also make a bath cover that would limit heat loss to the air.
Not sure if we can even buy a system like this in the US, I wonder how hard it would be to make one from scratch?
did you see the guy on youtube i think it was that built a hot tub out of an old horse trough? he set it up on bricks and put some charcoal underneath, several hours later perfect temperature!
also, along the same vein, the solar banks that people have set up to heat water or air. also on youtube.
Lots of ways to do this. Here's another commercial version that works with a bath size as well as the large tub they use, and can be run on gas or firewoodhttp://www.kiwitub.com/
In NZ lots of people have an outside bath. Most people just use a steel or cast iron bath and light a fire underneath. It doesn't take that long to heat (maybe an hour in winter). You can also use a gas stove. Some people get more fancy and build protected fireplaces with chimneys etc. A rocket stove system would be perfect.