Instructables user nagutron put together a guide on creating your own portable wood fired ocean hot tub. While this version requires an electric generator, a pump, and some welding skills, I’m quite sure you could do it with less.
The basic premise behind this wood-fired hot tub is that a heat exchanger, made from coiled copper tubing, is propped up above a wood burning fire. Water from the tub circulates through the tubing thanks to a generator powered pump, which then flows to and from the hot tub. The inflatable hot tub makes for easy transporting to whenever you might be headed.
(Images via Instructables user Nagutron)