Altho we use the Carnival Fun Points credit card on all of our charges with them and have not experienced this issue, there is this on the Carnival site....
"For your convenience, charges for most of your onboard purchases will be posted to your Sail & SignŽ 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, DiscoverŽ 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 & SignŽ 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..."