The pilot descended the helicopter out of mountainous terrain and then continued over a large mountain lake, cruising about 100 knots at an altitude of about 200 to 250 feet. The lake water was very clear and perfectly calm. The pilot looked inside the cockpit to check instruments, and when he looked outside and below the helicopter, he experienced a feeling of mental confusion similar to vertigo. At this point the pilot estimated that the helicopter was between 15 and 20 feet above the water. He pulled aft on the cyclic, the helicopter impacted the water, pitched forward, and sank. The pilot and his two passengers were able to egress the helicopter while it was submerged. Immediately after the accident the pilot told a park ranger who was on-scene that he was traveling a little low and thought he set a skid in the water. One of the passengers stated that moments before the collision with the water, the pilot had asked them if they wanted to get closer to the water and if they wanted to chase some birds.