The helicopter was being used in a program of disease eradication in cattle. After operating for about one hour, the pilot flew to a boat anchored in the river and hovered alongside it while signalling to the crew that the aircraft would return in about two hours. As the helicopter left the hover it was climbed to approximately 60 feet in a left turn through 270 degrees. The aircraft was then descended to an unnecessarily low altitude while gaining speed. Shortly afterwards, as the aircraft approached the bank of the river, the pilot realised that the aircraft was not responding to control inputs as he had expected. He applied more collective control in an effort to avoid flying into the water, but the rotor overpitched and the aircraft struck the water at about 40 knots. No defect was found that would have prevented normal operation of the flight controls or engine.