The private pilot was operating the floatplane under the provisions of Title 14, Code of Federal Regulations Part 91. He received clearance to takeoff from the west waterway. The airplane was on step, gaining airspeed, and the takeoff run seemed normal to him. The airplane was nearing takeoff speed and proceeding directly down the waterway, when it encountered a right quartering tailwind gust that lifted up the right wing and float. The airplane veered to the left toward a steep bank, and the pilot was unable to correct the deviation with the rudder. He did not feel that he could reduce power as he would slam into the bank. The airplane lifted off, but the float collided with the top of the bank. The airplane cart wheeled about 160 degrees to the left before coming to rest on its right side. It sustained substantial damage to the wings, fuselage, and floats. The pilot reported that there were no mechanical malfunctions or failures. Reported wind at the airport approximately 3 minutes after the accident was from 020 degrees magnetic at 3 knots, with no recorded gusts.