By Rob MacFarlane A raw-water cooled engine draws in sea water from the ocean, pumps it through the motor or heat exchanger to extract heat, then returns the water  overboard. On my boat, the cooling water is combined with engine exhaust gases through the exhaust elbow and wet-exhaust line. An issue with using sea water is there’s no control of what's in the sea water being pumped in — all sorts of detritus can be included, including small fish, sea weed, debris, bits and pieces. The engine cooling system's first line of defense is the lowly sea water strainer —…