When my mechanic and I went through the engine after a serious overheating issue, all but one of the cooling ports were blocked with salt. After the various cleaning and repairs were done, including him manufacturing a brass sleeve from a bar of bronze (for the alternator shaft having been damaged during the overheating), he mentioned putting a coolant/heat exchanger system on the boat to prevent any future corrosion from raw water in the engine.
He replaced the raw water pump with a dual water pump. Raw water in, pumped to a heat exchanger (on the forward engine bulkhead), with the coolant pump running the other side of the pump. There is now an expansion tank on the starboard engine bulkhead, with the various hoses doing their thing. I should say that the heat exchanger is only approx 12 inches long by 3 inches in circumference, and the expansion chamber is 4 inches in circumference by 8 inches long, so that fit easily in the engine compartment with plenty of the original working space. Even the hoses are fair to navigate around. I can send photos if you like.
The fact that he has 50 years of marine diesel and gas knowledge behind him, and that he had done over one hundred such retrofits, made it sound pretty good. This fellow has built his own tractor from a VW diesel, as well as being one of the go to marine mechanics in the area. At 71, he is trying to retire, but probably won't until he can't lift a spanner anymore.
That he charged me $20/hour for his many hours of labor, was greatly appreciated as well. Old school guy who loves his life.
I've also replaced the starter with a new Mitsubishi starter for $195 Cdn., as well as added a temperature gauge in the cockpit. I'm planning an oil pressure gauge as soon as I'm ready to run the wire again.
Cheers...