N83007

While practicing step taxiing on the river the pilot under instruction attempted to transition from step taxi to plow taxi. During the transition the right wing dropped and contacted the water, which damaged the right sponson. The flight instructor added power and proceeded to takeoff. The airplane climbed to 10 feet above the water when it then started a slow uncontrollable turn to the right. The airplane lost altitude and impacted the water. Both pilots egressed the airplane unhurt. The airplane's hull/fuselage was substantially damaged and it took on water.The pilot reported no preimpact mechanical malfunction or failure with the airplane that would have precluded normal operation.