When I was in college at WWU in the mid 1980's I had friends that rented a house on Lake Padden. They built a floating hot tub that looked a lot like this only it had a wooden deck and hull. They built a tub out of fiberglass, packed styrofoam around it and built a wooden hull around that. They built a stove out of an old beer keg and placed it in the bow with some rocks around it to hold and transfer heat. It had a big chimney sticking up like the one in the picture. The stove area was partitioned off from the main tub with wooden slats. At the stern was an ancient outboard motor and they would putter around the lake with it. They would fire up the stove and put sheets of styrofoam over the tub to get it heated up. It would get the water hot enough you had to use a bucket to pour some lake water in to regulate the temperature. We would motor around the lake (slowly, it was pretty slow) and get drunk. When you were sitting down in it your head was popping up out of the water right at the level of the lake except you were warm. Very awesome for cruising around on a cold night.
The guy who owned the floating hot tub took it down to Seafair for the hydroplane races a couple years. He had this rickety old trailer he transported it on that he towed behind his old Volvo 122.