Here's a bit of history I never really knew about. I've seen the concrete floating Mulberry harbours in Normandy, pulled there for the landings. An amazing sight that is quite complete even today. This is something else. https://www.youtube.com/watch?v=QJcbZ9lV5Gs Concrete ship - Wikipedia, the free encyclopedia
Yeah, but they didn't live a long useful life as ships. Most all of them were retired much earlier than their steel brothers. For example: Faith was launched March 18, 1918. She cost $750,000 to build. She was used to carry bulk cargo for trade until 1921, when she was sold and scrapped as a breakwater in Cuba.
The only cement ship I've ever seen was south of Bimini, it ran aground in about 12 feet of water, and became a target for bombing runs for the Navy out of Key West. It's long since been fished out, but back in the day, we could always count on picking up 12-25 lobsters every time we stopped. I think it's still there...haven't been to Bimini in 20 years.
There is a similar breakwater made of ww2 concrete ships at the mouth of the Chesapeake Bay at Kiptopeke. Pretty good at holding fish!
The WWI Sapona is still there indeed and still a great snorkeling spot. Slowly falling apart but enough left for people to climb on top and jump in the water,