The airline transport rated pilot was performing his third takeoff of the day with a full load of chemicals on an aerial application flight. On initial climb the helicopter did not have sufficient power to maintain the climb and began to settle. The helicopter settled into a lake in approximately 5 1/2 feet of water. The helicopter remained upright and sustained substantial damage to the drive train and rotor system as well as extensive water damage. The pilot, sole occupant, was not injured and was able to egress unassisted. The pilot stated in the Pilot / Operator Accident / Incident Report that the accident could have been prevented if he would have paid more attention to flying the helicopter than being concerned with mixing and loading of chemicals.