Depends... For example The Sovereign of the Seas (Royal Caribbean) was at Port Canaveral the day of Hurricane Charley. They were forced, by the port authority) to leave the port by 3pm (or they would have to ride the storm out there...naturally they left). Because they were to have left at 5pm they left a lot of people behind. Those that could be sent on to the next port were, those that could not make it at all were remibersed and then some. Remember the airport was closed (or closing down) as well so there were a lot of travel issues.
Cruise ships are worth a lot of money and the Captain is going to steer as far away from the storm as he can. For your safety and the safety of the ship. Ports can be changed on a moments notice, ittinerary could be altered greatly. A few years ago I believe it was the Magic (could have been the Wonder) tried to get around Hurricane Flloyd and it changed course so they eneded up just trying to out run the storm... They never made it to any ports, it was a cruise full of very rough sea days. Everyone on board got their money back as well as another cruise for free.
There really are a lot of variables... there was another Hurricane a few years back were the ships couldn't get into Port Canaveral (I think that was Flloyd as well) and they ended up having to dock I believe it was in Ft Lauderdale and bus everyone back to Orlando (will bussing everyone that was getting on that day from Orlando to Ft Lauderdale).
All effort is made to get everyone where they need to be safely and to depart on time... It really ends up being a case by case endevor.
They know when most people are flying in. Even if you don't book your air through them, I think they can note your flight numbers with your reservations. The DCL reps in the airport are all over the place on the day of the cruise. They should also have a home phone # with your reservation. They notify the travel agents, too, I'm sure.
It's probably also a good idea to check in with the cruise line yourself if there's any question as to the weather and your ability to board the ship. You may find out when you get to the port and they ask you to get on a bus. In my several years of being a cruise junkie, I've only heard of one other instance where a DCL ship had to board elsewhere. This past weekend was the second since 1998, so it's very likely that you'll be boarding in Port Canaveral as planned.
Once the cruise is underway, they'll keep you informed of any changes in the itinerary as soon as they decide.
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