His fishtail was flexible, designed and shaped like the lower half of a goldfish fin.

It used a wooden leading edge frame that tapered to nothing by the end, and a very flexible plastic panel for the innards of the tail. He moved the boat not by rapid pumping of the tail, but by gradual pumping over, and giving the fin time to change it's curve into propulsion. Then pumping it over to the other side. Ie, he moved like a koi, not a trout.

It probably would work well with a kayak, within it's own limitations. Ie, it's probably a dandy system for calm waters with a little depth (the fish tail sat something like 2' deep). Which is exactly where he described using it.