Carbon fibres themselves may be corrosion resistent, but the fibers by themselves are like a fabric. If you want a solid part instead of cloth, you need to encase the fibers in a resin. Imagine it like a piece of cloth soaked in beeswax or candle wax: it is solid like the resin but if you pull on it, it has the strength of the fibers of the cloth.

The resins used for carbon fibers are usually very bad at contact with water over long periods of time. Even those in aerospace applications require coating/paint if exposed moisture over time. It’s a plastic, even the best ones don’t do so well in water after a few months.

Furthermore, the damage that moisture does to the resin can be difficult to detect and even more difficult if not impossible to fix. It requires clean rooms, skilled labor and machinery that you don’t have in the middle of an ocean.

Then take iron corrosion: it is easy to spot by naked eye, it may not be easy to repair, but it is relatively simple to “halt” further damage by removing the rust and adding new paint.

Don’t get me wrong: carbon fibers are amazing, but sometimes the “boring” solution is best.

PS: steel alloys and coatings can be amazingly high tech too, it’s amazing what can be engineered.