Back in my intemperate youth a bunch of us did a boundary waters/quetico canoe trip. It was rainy and cold and due to a series of unfortunate events, most folks had lost rain gear and tent rain flys. We decided to pull one of the aluminum canoes up onto a rock shelf, fill it with water, and light fires under the bow and stern. It was actually kind of amazing... two of us could soak at a time, and it got hot enough that we had to add lake water to cool it down.
We paddled for another few days with an obviously deformed and dented canoe which large scorch marks front and rear. When people asked what happened we told them we got struck by lightening....
We ended up having to buy the canoe from the outfitter... but, due to said unfortunate events, that was almost certainly in the cards anyhow!
I jumped into a lake in the boundary waters in MN in early June to remove the filth. It was really cold, it snowed the next day on my birthday. We called it fumunda cheese, same stuff. Nasty after a week. When your in a tent with an unbathed human for 2 weeks, I found that sleeping in a canoe was preferable.I used to just bathe right in the mountain stream. That was pretty chilling, especially on an overcast day. But worked well for taking off the edge off the fromunda cheese.
A few years ago I tried out a a solar shower (bag that lies out in the sun to heat up, with a cheap shower nozzle.) That is about as close to a hot tub as I can imagine in the backcounty!