The aircraft had been refuelled from drums at a refuelling point located at one end of the lagoon. The refuelling point was surrounded by tall trees, so the pilot decided to hover taxi out over the lagoon and to execute a takeoff from a more open area. The takeoff had to be initiated with a slight tailwind, and the pilot intended to turn and to depart upwind after the aircraft had obtained translational lift. As the pilot initiated the takeoff, the aircraft experienced a tailwind gust and began to lose height. In an attempt to check the descent the pilot overpitched the main rotor. The right skid contacted the water surface and the aircraft simultaneously pitched forward and rolled to the right before coming to rest on the lagoon bed. The two occupants evacuated the aircraft without injury.