corrosion can get nasty in salt water.
look at the exhuats manifolds if they are weeping at all on the outside chances are they are pissing water on the inside.
After a while with raw water cooling the salt eats away the block. Its take as while but it can happen. My old engien was flushed pretty regularly (by eth previous owner and myself) and the block was starting to get eaten away. Not to a poitn where it was problem but if it needed to be bored again i doubt it could go out to 60 over.