Most marine engines both outboard and inboard use the available water around the boat to cool the power head. The most common way to go about this is to utilize a pump to draw water through a strainer either on the hull bottom or built into the lower unit. The water is sent into the engine to keep the power head, exhaust passages and gear cases operating at the correct temperatures.
The pump can be an internal part of the lower unit driven by the unit’s drive shaft or it can be an engine mounted assembly, typically driven by a belt. They use a rubber impeller that can be a positive displacement type or a vane type that runs as a positive displacement pump at slow speeds and a centrifugal pump at higher speeds.
As a vital part of the propulsion system, the pump needs periodic service. The impellers should generally be replaced every other year to ensure the uninterrupted flow of cooling water to vital drive system components. Occasionally, other components of the pump may need replacement due to wear. If the boat is used in silty or sandy conditions, or is run for many hours (300 plus) per year, more frequent water pump service is appropriate.