The pilot departed for a flight over the open ocean. About 1 hour into the flight, the pilot descended the helicopter to sea level to attach a GPS buoy to a log. After attaching the buoy and as the helicopter was climbing through 400 to 500 ft above the water, the pilot heard a bang come from the area of the rotor head, followed by the cyclic control shaking. The pilot contacted support vessel personnel, notified them of the issue, and proceeded in the vessel's direction, but the helicopter continued experiencing intermittent flight control problems and was unable to reach the vessel. The pilot attempted to deploy the inflatable floats; however, they did not deploy. The pilot subsequently initiated a ditching, and the helicopter impacted water and rolled inverted. The pilot and passenger egressed the helicopter, which then sank, and the helicopter was not recovered.