The aircraft was being demonstrated for a potential sale. The pilot reported that he was demonstrating a high speed taxi towards a boat ramp when the aircraft struck a submerged object and cartwheeled sideways. However, the passenger reported that the pilot was demonstrating a touch-and-go. As the pilot applied power for the takeoff, the aircraft commenced porpoising. On the third porpoise, the nose pulled into the water and the aircraft waterlooped to the right, tearing off the left float. The passenger had held a foreign commercial pilot licence but was not type endorsed or experienced in water landings.