When I had my Blue Water 40' with twin Chrysler 318 - 240hp, the exhaust manifolds were fresh water cooled. I had a leak in the block off plate / gasket between the manifold and riser that would eat about a quart of antifreeze per hour. Only leaked under pressure but would allow enough air into fresh water side so never pulled back from overflow tank. I made new block off plates and gaskets to go under the 4 risers ... no more coolant issues.
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