The helicopter pilot stated that he was spraying pesticide over a lake, adding that weather conditions were overcast skies and calm winds. He was spraying from east to west and west to east, and trying to keep his altitude between 50 and 200 feet above the water, and speed of about 30 miles per hour. He stated that the spray runs were about a mile. He was finishing up his 14th takeoff and landing for the day. He said the lake was glassy and that it was difficult to judge his altitude; he then flew into the water and flipped over. He reported no mechanical problems with the helicopter.