According to the pilot, he landed in order to examine his helicopter due to the intermittent illumination of a warning light. After landing, he reduced the throttle to "70 percent," exited the helicopter, and began a visual inspection as the helicopter continued to run. During the inspection, the helicopter initiated a takeoff, which the pilot could not arrest from outside the cockpit. The helicopter climbed to approximately 150 feet altitude and flew about 1,800 feet before it descended and collided with water. The pilot was seriously injured during the takeoff, and the helicopter was substantially damaged during collision with the water. The pilot stated that he failed to secure the collective control, and added, "There was nothing wrong with the helicopter."