Yeah, I’m thinking you accidentally got some water in there while spraying water through the tube, etc. Looks like a fresh water motor, so I don’t think corrosion/leakage is an issue.
That plug that looked like someone put it there might be just that….There were some modifications prescribed for those engines used in cooler weather and idled extensively that included plugging that little passage.
It is not uncommon to see different conditions in the combustion chambers…Those engines lost their idle quality when all the overboard drains were plugged and OMC attempted to recycle the unburnt/puddled fuel/oil.
I guess if it was me, I would check the gasket surfaces, and if they are OK, just replace the head gasket and reassemble…I don’t think you will have any further issues….There would have been evidence of rust and water inside if this condition existed before you worked on the engine…
.
.