The owner pilot and his wife flew to Cape Naturaliste where a water landing was carried out and the aircraft was taxied on to the beach. On departure the aircraft was positioned for a water takeoff into a 10 knot southerly breeze on a slight sea. The pilot reported that the aircraft lifted off after a few bounces at the end of the water run and was held level to build up climb speed. Shortly thereafter the aircraft appeared to yaw violently to the right, rolling to the vertical. The right wing entered the water and the aircraft cartwheeled coming to rest nose down and inverted. The pilot was flung clear and his wife was able to extricate herself from the wreckage. Both were supported in the water by their life jackets while awaiting rescue. The aircraft sank and was not recovered.