Now that I've looked at the shop manual, I am sure that it is a 2 QM..There were two models, a 15 horsepower and a 20 horsepower.. really a tough engine.. If you click on "Owner Resources" in the site, then "General Downloads" you can find the service manual for the engine .. Since that engine has an aftermarket fresh water cooling system, the exact cooling circuit will probably not be in the book, but it is still pretty simple .. there are two pumps, one for seawater, which pumps directly to the heat exchanger and then to the exhaust outlet; the other circulates the engine coolant from block to thermostat then either back to block or to the heat exchanger and back to block..
EDIT: There were also 2QM 20 H and 2QM 20 Y models.. yours really looks like a 2QM20 H model with an aftermarket fresh water system added.. If you look at the 2QM 20 Y model (factory fresh water cooling) you can get an idea about how the water systems flow..