Please careful when using a debit card.... If my understanding is correct, when you use them, the bank will automatically tie up a certain amount (depending on length of cruise) from your bank for up to 2 weeks after the cruise is over. Also, I'm not even sure that Carnival will accept a debit card. See below....
If planning on using a credit card, please notify your bank of your plans and where you could be charging, i.e, San Juan, St Thomas etc so that your card is not rejected while on vacation.
Below is from the Carnival website....
Deposit with a Credit Card
Credit Cards are accepted as a form of deposit to open a Sail & Sign Account at the start of the cruise. The following credit cards are accepted:
The American Express "Persona Select" is not an acceptable credit card.
Regular ATM debit cards are not acceptable.
Guests may use a checking account debit card as long as it is a registered MasterCard or Visa card.
A guest traveling without his/her parents cannot use the parents' credit card even with the parents' written permission
The cardholding guest must be an authorized signer by the card issuing bank.
Upon completion of the Sail & Sign Application Form, the cardholder agrees to pay for the individuals indicated on the form; it is not necessary for these individuals to be booked in the same room as the cardholder.
Important Note: A hold will be placed on the credit card or checking account debit card at the beginning of the cruise as a deposit for an onboard purchases. This will reduce the amount of available credit on the credit card or restrict the availability of cash in the checking account. The hold will be released and actual charges will be billed at the end of the cruise.
The following dollar amounts, per guest, are used as a deposit and placed as a hold on the credit card when the Sail & Sign Account is opened:
2-4 day cruises: $100 deposit pp
5-8 day cruises: $200 deposit pp
9 + day cruises: $350 deposit pp
Hope that helped....