Rubber Dockie via eBay offers the Rubber Dockie 18- x 6-Foot Floating Water Pad for $379.99 with free shipping. That's the lowest price we could find by $63. It comes with a 12-foot bungee tether and can accommodate up to six people.
I saw it and immediately thought ooh I'd buy this. Then I saw the price. If that zero wasn't there I would be getting it.