Most inboard boat engines are cooled by raw water from the river, lake or sea. It comes in through a through-hull fitting, passes through a strainer that catches weed and rubbish, and is pumped by a raw water pump through the heat exchangers, where it cools the engine's coolant, then out with the exhaust. Like a car, the engine itself is cooled by coolant in a closed circuit.

In this video I explain how a Jabsco pump, and indeed any flexible impeller pump, works, and show some of the problems with priming the raw water side of the cooling system.

An outboard's water pump has the same kind of flexible impeller, down in the lower unit.