The helicopter pilot reported that, while making a video of a boat, he flew the helicopter with the doors removed about 20 feet above the water, ¾ mile offshore, and saw seabirds ahead crossing from his right to left and thought they would be no factor. Shortly after seeing the birds, he initiated a right turn to follow the boat, and while he was looking over his right shoulder, he felt an object strike him in the back of the head. While dazed from the head strike, he flared to reduce speed, and the helicopter settled into the water. He then applied left lateral cyclic to stop the main rotor blade's rotation, and the helicopter rolled left and inverted in the water. The pilot reported that he believed he had been hit in the head by a seabird. The helicopter sustained substantial damage to the fuselage and main rotor blades. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.