[ndutton]

Originally Posted by Funky cat

Neil, the boat is in a freshwater lake in Canada, I fail to understand what locations have to do if the system is working properly or not.

If the boat is in the snow belt it brings in the very real possibility of improper winterizing and the resultant freeze damage. Based on your information a cooling system pressure test is indicated, preferably block and manifold separately. I wouldn't spend a second looking elsewhere until the engine passes this test.