You back this dune buggy down to the water's edge and suddently it gives birth to a 14-foot fiberglass boat. After the cruise or waterskiing, when you're back ashore, an electric winch hauls the boat aboard its wheeled partner. Dune buggy and boat are powered by same 50-hp outboard motor. The buggy has standard Volkswagen running gear and an infinitely variable hydrostatic gearbox of the type used on high-power garden tractors. The prototype, a…