The concrete isn’t the problem. Like mentioned above, the sealife growth is. Also, metal and moving mechanicals are savaged by seawater (and the sealife growth). Keeping things working on the surface of the water is difficult and expensive. Water pressure makes that even worse. Maintenance requires divers which are likewise very expensive.

Every time I see these “We’ll do X in/around the ocean” projects I think, “These people have not spent a lot of time near the ocean.”