Lots of Marine engines purposely block of portions of the cooling system. I'm gonna have to think and post to sort this out...
All of those openings do connect so the only reason to block ports is to give a flow direction. It looks like the blocked port is the one directly under the out let nipple for the coolant. Is that correct?
I assume that is made to re-direct the coolant to take a longer path, and not simply blow thru the cylinder.
What might help is drilling an 1/8" relief hole in a new gasket, and test run it. What I mean is a 1/8" hole in that blocked port (gasket) to allow air to escape, and still maintain MOST of the "suspected" purpose for blocking it at all.