The team behind the six-year-long project has a pretty ingenious setup worked out. The Caddy's original 472-cubic-inch (7.7-liter) V8 both provides propulsion and heats the water. The interior is entirely replaced with a watertight, fiberglass tub that includes working jets, and the controls are all done by hand.
As if just building a mobile hot tub isn't enough, the team behind the Carpool Deville plans to take it racing too. Specifically, they intend to go to the Bonneville Salt Flats later this year to make a top speed run while immersed in water at over 100 degrees. They even have a roll cage all set to install to meet the safety requirements there.
Everything appears to be all set to make the run in August. The team recently completed a successful Kickstarter campaign that raised $11,251 from a $10,000 goal to cover transportation expenses. Check out the Kickstarter page for a complete history of the whole project, including a previous attempt at making a carpool, and scroll down to watch their promotional video.