Mercury - Water Pump Impeller 15-20hp FourStroke EFI Series Durable Marine Rubber 8M0135803. Overview. Anyone who's spent a few seasons on the water knows how important steady cooling is for an outboard. When the impeller is healthy, the engine settles into a calm, confident rhythm - the kind of smooth flow you feel through the helm as much as you hear in the exhaust. The Mercury Marine part 8M0135803 Water Pump Impeller brings that kind of reassurance. It's a small component, sure, but it carri