Marine heat exchangers are the greater part 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 caustic a sealed mixture of distilled water and antifreeze may cool the engine. 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 from beginning to end the heat exchanger 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.
To protect the marine heat exchanger from corrosive salts, a sacrificial zinc anode is screwed into the heat exchanger. This anode must be periodically replaced as part of regular maintenance. Because the water the boat floats in may be contaminated with floating particles such as wood or Styrofoam balls the well considered boat will have a filter (often stainless steel mesh) to remove these particles before they are moved toward the heat exchanger. This filter must be periodically cleaned or else the flow of water to the heat exchanger will become obstructed and the engine will overheat.