The helicopter lifted an external load of 485 kg from the deck of a vessel at sea to transport it to land. After lifting the load, the helicopter gained climbing speed and began to climb to 300 ft AMSL for the cruise. Once at 300 ft the load was reported to be stable, the PIC then handed over control to the co-pilot. Five seconds later during a left turn at about 80 kts the external load released and fell into the sea. This was confirmed by the occupant in the rear cabin. According to the crew at no stage during the flight have they activated the electrical or manual release of the cargo sling.