The amphibious airplane was about 300 feet above the ground, on short final to land on the runway, when the engine quit. The airplane landed in soft mud with the wheels extended, and flipped over. Postaccident examination revealed fuel in both wing tanks. Bench testing revealed the engine driven fuel pump would not pump fuel, and drew air into the pump when primed and operated. Disassembly did not reveal any blockages, damaged seals, or sources of the air leak.