I've been thinking about this since Ian posted 2 years ago. I think the Nomad coil would work even better if fabricated inside of a washing machine tub fire pan.
Then use the pallet pool concept:
more info > link
Not practical for the grand, but would sure be nice at a festival or in my backyard.
Finish it up with a 5 gallon bucket sand filter and you have a great pool. You may also need a Honda EU generator, electric pump, etc.
I have been thinking about building a stock tank version for use in the backyard. I don't want a Jacuzzi, but like the idea of a warm water tank to lounge in.