I assume corrosion is to blame? Crazy how much ocean facing stuff is still done with painted steel. You'd think aluminum and carbon fiber or even plastic would be making strides but it's still the iron age in many ways it seems.
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.
https://www.popularmechanics.com/science/a60687211/titan-sub...
Also, I am thinking about all the ocean factors beyond salt corrosion. There's tons of crap in the water beyond salt and minerals. Like fine grit suspended in it. Plus the tidal forces etc.
While rust can be a problem it can be mitigated. Also steel is easier to repair than many other materials (welding).
BTW. Aluminium does suffer from corrosion as well. I used to have racing bike, the wheel nipples (these connect the spokes to the wheel rim) used to corrode to the point where they would fail, which meant I would end up with a buckle. I ended up having both wheel rebuilt with higher quality brass nipples.
Plastics under time also suffers from a different set of issues. Plastics can become brittle. Anyone working on old computers (especially macs) can attest to this.