I agree with the previous posters. I had a similar incident happen to me last year where someone across the country tried to use my card to buy something online and add it to a PayPal account. I've used this card online for years with no problem, so I have no idea how the fraud department caught this, but I'm glad they did. I'm still not sure how the number was stolen, but obviously, it was. I have a suspicion that it was when I bought from a small Web site that I'd never used before, but I can't confirm that. No big deal, though. The credit card company canceled the card and overnighted me a new one.
Could someone have accessed your safe? Sure, but it's unlikely. There would only be two ways into it: with the card you used to lock it and by a crew member. And considering how much cash and valuables are in safes all over a ship, I'm betting that only a limited number of ship's personnel have the ability to open them. Anyone who could open those little boxes would, on any given cruise, have access to hundreds of thousands, possibly millions, of dollars in cash and enough credit cards to buy a small island.
There are many easier ways to get your card info than that