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.
There are 2000 year old Roman concrete piers that are still just hanging out in sea water. So it’s possible if you find the right mix.
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.
Really good points. I was only thinking of the structure of the concrete… Sea life growth is a whole other ball game!