Your engine’s cooling system is highly critical; the combustion process produces an enormous amount of heat, and that heat must be removed or dissipated. If heat is not removed, cylinder and engine block damage may quickly result.
Heat exchangers cool the engine, but work a little differently and more efficiently than a simple raw water cooling system. A heat exchanger consists of a series of tubes, or coils, that fluid flows through. This fluid is enclosed (either cooland or distilled water). Additionally, marine heat exchangers are encased by a jacket, through which raw water (fresh water or sea water) flows through, cooling the enclosed fluid, removing the heat, and exiting the system through the exhaust. The minerals in sea water mean that scaling, or build up, can occur, which is why heat exchangers usually require periodic maintenance. Mineral build up drastically reduces the efficiency of the heat exchanger and thus, the cooling capacity.
This is also why a blocked strainer or faulty impeller quickly damages an engine. When blocked by debris (especially plastic bags), without the raw water to cool the enclosed system, the engine quickly overheats, sometimes in a matter of mere seconds.