I've seen folks do this by making a copper coil, (maybe 20 feet of 1/2" copper tubing wrapped around a bucket) placing it in a fire and pumping water through it. They made a water trough hot tub (a metal 2 person deal) and it worked great. It was pretty simple - I believe they just used a drill attachment style pump, pumped water out of the river with it and then circulated it from the tub, trough the fire and back into the tub. took like an hour to get it too hot and had to add cold water.
"If you dont do it this year, you will be one year older when you do"