crusing on 9/1 I used my visa debit card. Be carefull. They put a hold on my account for the items I used the sign and sail card. Then they also took the amount out of my account without removing the holds they also had. Ended up with me having an extra 1200.00 that I could not use until they lifted the hold. The holds were on till 9/23. Had to call carnival 2 days to get the holds taken off sooner and lots of arguing with them. Had to have the bank also called. The only was it was resolved was to insist on talking to a supervisor to get it fixed. The bank said they see this all the time ( double billing ) and advise people to use cash instead.
Absolutely use a Credit Card with enough available credit above the Deposit Hold. NOT a Debit Card, Besides the hold for cash funds, If you use a regular Visa/MC/Amex you can later dispute any wrong charges. You can still pay your account with cash, or a different card the last day. You don't want a hold on your bank account.
Hold amounts are 2-4 day cruises $100, 5-7 day $200 per person on the account being secured by the card.
I recommend taking 2 cards if you can. Would depend on the credit limits and credit available. I secure account with one and keep one for travel emergencies.