The pilot reported that at the time of the accident he was practicing a “water ski” maneuver on the open waters of a remote river in his tailwheel-equipped airplane. The water ski maneuver involves the airplane's main wheels touching the surface of the water while maintaining flying speed. The pilot added that while maintaining about 90 knots indicated airspeed, with the main landing gear wheels “water-skiing” across the surface of the water, he recognized that there was additional drag being placed on the wheels, and the airspeed began to decay. In response, he added engine power, but the airspeed continued to rapidly decay, and the main wheels subsequently settled into the water, and the airplane nosed over, sustaining substantial damage to the wings and engine mounts. The pilot reported no mechanical problems with the airplane that would have precluded normal operations.