Although not quite a hundred dollar idea. I thought there might be some parts of this underwater hot tub that could actually fit into the concept of the site. After seeing my Aunt and Uncles wood-fired hot tub I came up with this idea.
The basic idea is for people who live on or near large bodies of water. Slightly offshore they could install this hot tub and be able to sit in a small body of water inside of a much larger body of water.
A simple pump unit would feed the water heater and cycle hot water back to the tub. A small onshore fire would heat the device from shore or on a floating platform. I’m sure there would be an easy way to siphon water to the tub too.
Built in pumps would enable people to be able to soak and exercise at the same time. Much like what PlayPumps has going on.