The pilot of the amphibious airplane was conducting a takeoff from a lake when he rejected the takeoff because there were power lines and a bridge in the direction of takeoff and he was unsure whether the airplane would be able to clear them. During the rejected takeoff, he reduced engine power; however, the airplane became airborne. He initiated a slight right turn toward more open water. After touching down, the airplane "water looped." The aft portion of the fuselage was substantially damaged during the accident sequence. The pilot reported that there were no preaccident mechanical malfunctions or failures of the airplane that would have precluded normal operation.