Marine heat exchangers are the most common way to cool a boat's engine, using the lake, river or ocean water in which the boat floats. Since this water may be corrosive the engine may be coole by a sealed mixture of distilled water and antifreeze. Heat from the water-antifreeze mixture is then transferred to the ocean (or lake or river) water which flows into a heat exchanger. The water-antifreeze mixture runs through it dumping heat, but remaining separate from corrosive salts and chemicals found in the water the boat is floating in. If the ocean water eventually corrodes and ruins the heat exchanger it can be replaced at a fraction of the cost of replacing the engine.