HELLO,
WHATEVER YOU DO, DO NOT USE A DEBIT CARD. On my second cruise I used my debit card. I thought once they had finalized everything on my day of departure that was it. Nevertheless, once I arrived at home I called the bank and low and behold Carnival Cruise Line had charged my account twice for all of the same purchases.
While your on the ship the money is taken off as if your doing a transaction in the store, it admittedly comes out of your account. Once off the ship, the same amount that they charged you for is the same amount that is put on hold. For example, my total purchase on the ship came to $278.50,when I called my bank two transactions were showing on my account with the same amount. One was noted paid, while the other had a hold on it to be released within three to five business days.
I have learned, always bring cash for your sail and sign account. Due credit or debit on shore it's so much easer.