Engines utilize one of two cooling systems, or variations there of. Simply, there is a Seawater Cooling System, sometimes called Raw Water system and a closed Cooling System. Seawater systems are just that… they utilize the water that the boat is operating in to cool and lubricate the drive and engine. Water is drawn in trough the stern drive unit and circulated to, and trough, the engine and its components. The heated water is then returned overboard trough the exhaust system. Closed Cooling systems are aaaaaaactually two systems working together, a seawater system and a closed circuit system consisting antifreeze, very similar to an automotive system, with out the radiator but using a form of heat exchanger.