Had you EVER used that card before? Just because the last time you used it was to set up your sign and sail account does not mean that was when your information was stolen. It doesn't work that way.
Had you ever used it at a bar or restaurant? Bought something online? Paid a bill using your credit card? Threw away your credit card statement instead of shredding it?
Your credit card information can be stolen in many ways. The waiter could take your card to settle your bill and when they are out of sight run the card through a small scanner and retrieve every bit of information on it. Or they could just write it down. The company that you bought your Miracle Slicer Dicer from had bad online security and someone hacked into their system and retrieved all the credit card information from their database.
This information could be floating around for months until someone buys it or decides to use it and then the invalid charges show up. The information could of, and probably was, stolen weeks or months earlier.
Blaming the "last" place you used your credit card is just a way to create convenient scapegoat and is usually not correct. The only way you would know for sure is if that was the only place you had used your card.
So: Don't blame Carnival.
BTW: A month ago I had to cancel my card because someone in Ireland tried to buy $1,200 worth of liquor. It was caught immediately and new cards were issued. When the information was stolen could have been any time.