It has a closed cooling system with a raw water heat exchanger, right? If so then your closed system is pretty much the same as on a car, water pump circulating your antifreeze/water mix through the engine and the inner core of the heat exchanger, and a second loop that takes raw water (the water the boat floats in) and pumps that through the heat exchanger and out, as I understand it. Here's a basic description - Marine Engine Cooling Systems - BoatSafe.com , and not trying to preach to the choir here, just trying to understand what you've got.

Seems to me that if you're getting coolant in your oil pan you've got a problem with the block unless I misunderstand your system.