The pilot stated that he had lack of appetite and sleep during the previous 3 days which made him fatigued. He awoke about 0400, and departed about 0430, for a cross country flight which was flown between 1,000 and 1,200 feet and 102 knots. About 1 hour 25 minutes into the flight, or at 0555, he recalled the sound of water hitting the helicopter and the next thing he remembers was being very still and quiet. He realized he had crashed, released his restraint, and called his girlfriend via cellular phone for help. A search was initiated and he reported being rescued about 1030, then airlifted to a hospital for treatment. He further reported there was no mechanical failure or malfunction of the helicopter, and the cause of the crash was "...due to fatigue."