It happened on an American-Hawaii cruise I was on (several years ago -- the cruise line has since gone out of business, unfortunately.) The ship was docked at the Big Island, and tied down with humongous rope cables but the wind came up -- nowhere near as fierce as the Alabama winds yesterday -- and two of the cables snapped. If anyone had been close to the cables when they snapped, the force could have killed them. So the captain took the ship out of port a few hours early for the safety of the ship, dock, and the passengers.
DH and I were still in port and it was rather eerie to return to the dock about an hour before we were scheduled to sail, and find that the ship had sailed without us. A worker in the terminal told us what had happened, and reassured us that the ship would return for us.
How many other vessels have broken loose in high winds? We have no idea because the press only reports when it's Carnival.