The water pump on an outboard or I/O is the heart of your engine’s cooling system, allowing raw water to circulate through the powerhead to prevent overheating. A key component of the pump assembly is the rubber impeller, which spins inside the lower unit to provide the suction and pressure needed to supply water. Over time, the rubber can deteriorate and lose its elasticity, leading to a loss of pressure, and small pieces of the impeller can break off and be sucked into the cooling system. Internal gaskets, rings and seals can also break down, allowing water to escape inside the engine housing and, again, causing a loss of pressure.
In short, the water-pump system is something that needs routine maintenance—once every 2 seasons, according to most experts.
Watch the video on how to replace a water-pump impeller on a 200-hp Mercury outboard: