1993 Cummins 6BTA5.9 M2 Coolant issue.

Yes. Most all marine diesels have a heat exchanger system. The salt water from the raw water pump circulates in a separate closed set of hoses but pulls the engine heat and after cooler heat through the heat exchanger ( both salt and coolant flow through this) but the two liquids do not mix. The salt water is exhausted out the muffler system and the coolant keeps circulating now with it’s heat given to the salt water. <br />Additionally there is an accessory bottle that captures the coolant that expanded in volume from getting hot.. But when the engine cools the engine sucks the now cooler excess coolant back into the engine’s expansion tank ( where the radiator cap is).  This keeps the level of coolant to it’s max. Without it the hot expended coolant drains out and the expansion tank stays at a lower level. May be sufficient to keep engine OK but most of us like the extra coolant available.<br />This is usually a plastic bottle mounted near the front of the engine maybe on a bulkhead with a little tube connecting it to the expansion tank. <br />All this only works if the coolant cap (radiator cap) is working.  Without it the coolant leaks out one time and just stays at that level.  If you refill it it will just spit out. But may not be real noticeable.