The pilot was carrying out aerobatic manoeuvres at 1500 feet over the sea near Rottnest Island. As the aircraft approached the top of a stall turn, under zero gravity conditions, the engine stalled and the propeller stopped. The engine was not fitted with an electric starter motor and there was insufficient altitude available to complete a windmill start. The pilot carried out a successful forced landing in the sea. Subsequent inspection revealed that the engine driven fuel pump had either failed, or was unable to produce sufficient fuel flow to sustain engine r.p.m, due to a split in the rubber diaphragm in the pump. It is probable that loss of fuel pump pressure combined with the the zero gravity conditions at the top of the stall turn caused fuel starvation which resulted in engine failure.