The pilot and flight instructor were conducting a local flight in the float equipped airplane from a seaplane base. The pilot stated that there was a crosswind from the southeast and he planned to takeoff to the southwest with a 30° left turn while "on step" to avoid the lake weed bloom in progress. The pilot added that during the takeoff attempt, with the airplane "on step," he made a left turn toward an opening in the weeds, swung around a buoy, then the airplane capsized. A witness reported that during the takeoff, the left float was higher than the right and the right wing dipped down. The pilot reported to the responding Federal Aviation Administration inspector that the centrifugal force acting on the airplane and the left crosswind allowed the left wing to lift and the right float sunk into the water. The airplane sustained substantial damage to both wings and the engine mount. There were no reported mechanical malfunctions with the airplane that would have precluded normal operation.