The problem, interior corrosion build-up in exhaust manifold. Not enough threads in manifold exhaust coolant outlet to tighten down pipe fitting. Salt water leaks make a mess of cast iron.
When in doubt, pull it out and bring it to the workbench.
ugghh...
Hacksaw, pipe wrench and chisel make quick work of the pipe nipple.
Corrosion here too, quick cleanup to ensure exhaust gasket seals.
Used pipe thread tap to open up coolant outlet. Note that the drain port is completely blocked. A Dremel tool and wire coat hanger cleared it out.
A little fresh primer on manifold and hi-temp paint on exhaust pieces. Problem solved!
