I help build a cement fishing boat in Hawaii in 1967. what makes a boat float is not the material it is made of but how much water it displaces versus the weight of the boat. steel ships float same as concrete.
Steel and ferro-cement re-bar suffers from the same problem - salt water. It is very hard on iron based metals as the rust corrosion can be severe.
If you replace the re-bar with fiberglass re-bar, you avoid the structural rust that plagues ferro-cement. If you change the concrete mix to reduce water permeability you further extend the life. Trade off as it is a minor cost increase but significantly extends the life of the craft.
Yep.
The Chronicles of Thomas Covenant: granite ships.