Mercury - Water Pump Housing OEM Replacement for 25 & 30HP FourStroke Outboards 803750T05. Overview. If your engine isn't staying as cool as it used to, it might be time to replace the water pump housing - the heart of your cooling system. The Mercury Marine 803750T05 Water Pump Housing is built tough for 25HP and 30HP FourStroke outboards, giving you factory-quality performance and peace of mind. It's made from durable, marine-grade materials that hold up to years of heat, vibration, and water