There are several drain valves on this engine that you need to open. On mine it has a little clear 1/4 inch tube attached to each one. The heat exchanger is the part with the radiator cap. If the engine is cold it is safe to take it off for a look. You should have it loose for it to drain properly.There is no one drain valve like a car radiator. Your heat exchanger is the radiator. Also change the oil and filter. My 3GM30F calls for #30 diesel oil and the same for the transmission. I change both at the same time I do it at 100 hours or twice a year. Good luck
That derelict boat was another dream for somebody else, don't let it be your nightmare and a waste of your life.