I’m not talking about the heat exchanger, but an actual radiator like in a car
I've been a boat mechanic for over 35 years and I've never seen a radiator on boat engine. I'M curious why you ask. Are you thinking about heating the boat like a car? If so you take advantage of the coolant heat, before it's discharged overboard.
About the only application that would need one would be a swamp boat. Most marine applications use water from outside the boat to cool the engine and return it to the river/lake or other body of water. When operating in salt water special considerations are made to the engine alloys that come in contact with the coolant. But now always, for every rule there is an exception.
