Than automotive versions? No. Blocks for any application can have perfect or imperfect castings however. If a marine engine used sea water for cooling, they can get very thin in spots where corrosion has set in.......sometime to the point where the cooling passage corrodes completely through. If the marine engine used heat exchangers and had a sealed cooling system with the proper coolant mix, it in itself is no better or worse than an equivalent automotive block. Always have a block sonic tested if it going to be bored oversize regardless of the source of the block. Now, if that block sat in a bilge full of salt water, it may have some nasty external corrosion!