Fresh water (or coolant) is usually circulated through a boat engine by a centrifugal pump, the same type of pump that circulates the coolant in your car’s engine. Centrifugal pumps rarely fail, and when they do–indicated by water dripping from a hole in the bottom of the pump–they are simply replaced. But because they are intolerant of foreign matter, centrifugal pumps are not used as raw water pumps.
The raw water side of the cooling system is almost certain to have a rubber-impeller-type pump. Rubber impellers pass twigs and pebbles and small pilchards, but stop the flow of water to them and they shed blades like leaves in an October storm. What could cause the flow stop? An intake blocked with a plastic bag or other debris. An air lock resulting from heeling under sail. A closed seacock.
Checking the exhaust for spray every time you start the engine can sometimes prevent impeller damage -if you react quickly to dry exhaust. But despite your vigilance, sooner or later the raw-water pump will fail, and rarely at an opportune time. To avoid the collateral consequences of pump failure, routine impeller replacement is a good practice. Many boaters replace the impeller annually. How often you should replace yours depends on how much or sometimes how little you run your engine.