Water heater on a d318 marine?

Would anyone be able to tell me were I should tap into the cooling system on a D318 marine to connect to a water heater? Usually you hook a line to the pressure side and a line to the suction side of the circulating water pump. These lines, with hot coolant from the engine flowing through them, hook up to a heat echanger in a water tank and as you cruise along the water heats up, and you can have a nice shower or hot water to do the dishes. The suction side looks pretty easy because there is a pipe plug on top of the heat exchanger tank on the engine. On the pressure side I'm really not sure where I should come from and I dont want to do anything that will do any harm. The only places I can think of would be the opening on the head where the temp gauge is, two drains on the exhaust manifold, the drain on the side of the block, there is a pipe plug on the top of the adapter assembly,and the drain on the water pump looks like it would be on the pressure side.

I know that this is'nt the usual type of question for this forum, but what the hell, maybe someone can answer this.

The best place to connect, with the hottest coolant, would be the front of the head.

On the industrial engines the high water temperature shutdown is tapped into the thermostat housing where there is a small removable flange. This would translate to the adapter housing on the marine engine and there appears to be a pipe plug access. Seems as though this could be your source for hot water to run through a heat exchanger. I cant tell what the equivalent low pressure or suction side connection would be on the marine engine but there most likely is one.

I am told all the #12 graders had cabin heaters that were tapped into the cooling system. Can anyone tell me where the coolant lines for the heater attached?

Not all had heaters...they are an attachment. Connection was the small two bolt cover on the side of the thermostat housing and a pipe thread connection on the water pump suction line for heater return.