What is the purpose of a seawater cooling system's heat exchanger in a diesel powered ship?

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The heat exchanger's job is to remove heat from the engine coolant by transferring it to seawater. In a diesel ship, the engine generates a lot of heat as it runs, and coolant circulating through the engine absorbs that heat. The coolant then passes through the heat exchanger, where it gives up its heat to the seawater flowing on the other side. Because seawater acts as a large, effective heat sink, the coolant is cooled before it returns to the engine, helping keep engine temperatures within design limits and maintaining operating efficiency. The two fluids are kept separate by the exchanger’s design, so they don’t mix.

That’s why this option best describes the purpose: heat is transferred from the engine coolant to the seawater to keep temperatures under control and preserve engine efficiency. The other statements aren’t correct because they either suggest heating the coolant, imply the heat goes the wrong way, or refer to filtration, which isn’t the primary role of a heat exchanger.