I'll explain a little about the Sail and Sign card.
You will get this when you board the ship it acts like
an on board credit account which you charge drinks, souviners (NKOTB Cruise
Merchandise) and shore excursions too and its paid the night before or day you
get off the boat. Also it is used as a form of ID to get off and back on the boat at the ports.
A little more information off Carnivals website to explain how it works:
For your convenience, charges for most of your onboard purchases will be posted to your Sail & Sign® account. A required application form will be included with your documents. Registration for this program will occur at the time of embarkation. Carnival accepts
American Express, Visa, MasterCard, Discover® Network Card, Diner's Club, Optima Card and cash for deposit on your account. A deposit (ranging from $100 to $350 depending on cruise duration) will be required for cash accounts.
Third-party credit cards, Travel Funds Cards, Credit Card Gift Cards and Mini Credit Cards are not accepted on board.
If you are using a credit card or debit/check cashing card please be advised that there will be a hold placed on your account as a deposit for your onboard
purchases. During your cruise additional holds will be added if your Sail & Sign® account exceeds the amount of deposit Carnival has on file. If you presented a credit card, this will reduce the amount available on your credit card for other purchases. If you presented a debit/check cashing card, the hold will restrict the available cash in your checking account. All charges will be billed to the credit card or debit/check cashing card at the end of the cruise.
Please note that Carnival Cruise Lines will not be held responsible for any bank-imposed overage or insufficient funds charged on debit or credit cards.
Your Sail & Sign Card will serve as your Boarding Card. This card identifies you as a Carnival guest and must be carried with you along with picture identification when you go ashore. Because the ship is closed to the public while docked at the various ports-of-call, you’ll have to show your Sail & Sign Card and picture I.D. to the officer on duty in order to return on board.
NOTE: - Also note that I recommend using Credit Card if possible and second cash in the past ive read of people having trouble using debit cards and not receiving the hold on the funds back for up to 72 hours after they have departed the cruise.