The airplane was taking off when the engine quit at the departure end of the airstrip approximately 50 feet above the ground. The pilot landed straight ahead on a short gravel bar and nosed over into a river, with the carburetor remaining above water. The pilot said the front fuel tank was full, and selected. The fuel system remained intact. Disassembly of the engine revealed no mechanical, fuel system, or ignition system anomalies. No fuel was found in the main fuel supply line. A mix of water and fuel was found in the carburetor bowl.