They seem to be buoys hung on the side of boats to protect them against banging into things. I'm sure someone more nautically minded can give the exact term (or correct me if my guess is wrong). Until then I'll just call them "buoys".
I'd say "The protective covers for the buoys guard against external wear-and-tear".
It seems that it is the buoys being protected from wear-and-tear rather than the boats.
Edit - after a bit of investigation apparently they are called "fenders" in English, so substitute "fenders" into my translation above.