I was at a festival and had an amazing hot tub which was a very large blue plastic tub with pipes coming in near the top and out near the bottom. It was powered by two central heating radiators which we're leaning together with a small fire underneath. The water heater up, came out at the top of the radiator and up into the hot tub and cooler water ran out near the bottom and back to the lower inlet valve on the radiator. It was very portable and could be moved around on a trailer. Great temperature too.
