Mercury - Water Pump Pulley Cooling System Component Cast Steel Finish 865777. Overview. There is something comforting about hearing your engine settle into that smooth, steady rhythm after you push off from the dock. The cooling system plays a huge part in that peace of mind, and the water pump pulley is right at the center of it. When you're running a boat you love - one that carries your family, your gear, and your weekend plans - you want every part under the hatch to work without hesitation