The pilot reported that he was landing the helicopter on the shoreline of a lake, during low light, dusk conditions. During the approach to the shoreline, he began maneuvering to slow the helicopter down and descend over the open water and that his attention was fully outside of the helicopter, when he saw a light from the intended landing zone get brighter, and the area around the helicopter suddenly got darker. The helicopter subsequently struck the surface of the lake, rolled inverted, and sank in about 80 ft. of water. The pilot and the passengers exited the helicopter and were rescued by a good Samaritan before it sank. The helicopter sustained substantial damage to the main rotor drive system and tail boom assembly. The pilot reported that there are no mechanical malfunctions or failures that would have precluded normal operations.