The helicopter was engaged in a search and rescue (SAR) training exercise over the sea at night, in visual meteorological conditions. The task was to drop two small improvised marker buoys from the helicopter in flight and for the pilot to record the latitude and longitude of the buoys by using the global positioning system (GPS) fitted to the helicopter. Small boats were tasked to search for the buoys. As planned, the helicopter crewman dropped the two marker buoys through the left rear passenger window while the pilot flew at 100 feet radar altitude above the sea with about 15 knots forward airspeed. As soon as the buoys were released the pilot directed his attention to the GPS to obtain a fix. Then, just as the pilot raised the collective lever to climb, with the helicopter moving forward at about 15 knots, its landing skids, fitted with emergency popout floats, contacted the water and the helicopter nosed into the sea. The four persons on board, none of whom was wearing a life vest, were rescued within 20 minutes. There was no life raft in the helicopter. The helicopter was swamped before the pilot had a chance to inflate the emergency floats. The radar altimeter had been set for its warning light to illuminate at or below 100 feet above terrain or water. The pilot confirmed the operational serviceability of the radar altimeter (RADALT) enroute to the buoy drop site which was two kilometres out to sea. During the buoy drop and the position fix, the helicopter was facing towards land where the pilot could see lights. While fixing the GPS position of the buoys, the pilot was unaware that the helicopter was descending. The following factors were considered relevant to the development of the accident: 1. The pilot channelised his attention to the GPS while flying at a low height at night. 2. The pilot was not aware that the RADALT warning light had illuminated. 3. The visual cues available to the pilot were insufficient to enable him to monitor the height of the helicopter above the water.