Marine heat exchangers are the most common way to cool a boat's/ship engine, using the lake, river or Sea water in which the boat floats. Direct cooling of the cylinders and heads by sea-water is unsatisfactory because sea water will eventually ruin the cylinder block and heads. Since this water may be corrosive the engine may be cooled by a sealed mixture of distilled/Fresh water and antifreeze.
Heat from the water-antifreeze mixture is then transferred to the Sea (or lake or river) water which flows into a heat exchanger. The water-antifreeze mixture runs through the heat exchanger dumping heat, but remaining separate from corrosive salts and chemicals found in the water the boat is floating in. If the Sea water eventually corrodes and ruins the heat exchanger it can be replaced at a fraction of the cost of replacing the engine.