The pilot was departing from a gravel bar (1,200 feet long as estimated by the pilot, sand and gravel) on a Title 14 CFR Part 91 personal flight when the accident occurred. He stated that prior to the takeoff he examined the gravel bar for the firmest ground for the takeoff roll. After the tail came up, during the takeoff roll, the pilot realized that he was not aligned along his intended takeoff path. The pilot was not able to abort the takeoff roll and rolled from the gravel bar to the water. The airplane came to rest in the water inverted resulting in substantial damage to both wings and the empennage. The pilot verified that there were no mechanical anomalies with the airplane prior to the accident.