I had on board credit on Carnival. It's kind of like a credit card. On the last night of your cruise, check your 'bill'. They usually just deduct whatever you charged on board. (alcoholic drinks, specialty restaurants, spa, your daily tipping, etc.) You just sign a receipt and it goes on your 'room account'. They will ask you for a credit card, usually before the cruise. It all can be charged on this. I believe you can also pay cash on the last day for your room 'charges', but who wants to carry all that cash? Things will be easier for you if you stick with the credit card you originally gave them. Also realize the cruiseline will probably put a 'hold' for a certain amount of money, tying up some available credit.
Remember to notify the banks of your credit cards/debit cards that you will be using them on a trip. They will red flag your account if they see suspicious activity, ie. out of country charges.
You will need cash if you wash your clothes at a self laundromat and if you want to tip servers, baggage handlers, maitre'de's extra.
If the cruise offering the $350 credit is the cruise you want, book it! And spend it! You can't take it home with you.
Basically, when a cruiser boards a cruise ship, they will establish an onboard account with the cruise line for any and all onboard purchases. Should the cruiser purchase any drinks, spa treatments, specialty restaurants, souviners from the gift shop and stuff like that are then charged to the cruiser's onboard account. At the end of the cruise, the cruise line will charge the onboard account to a credit card the cruiser gave the cruise line at the start of the cruise. An onboard credit is simply an amount the cruise line will not charge the cruiser's credit card. For example, cruiser A takes a cruise and has a $100 onboard credit. At the end of Cruiser A's cruise he or she has run up $500 on his or her onboard account. With the onboard credit, the cruise line will now only charge $400 to cruiser A's credit card.
Majesty of the Seas - 10/03 & 02/07 & 11/08
Enchantment of the Seas - 10/04 & 10/11
Mariner of the Seas - 10/05
Vision of the Seas - 10/06 & 09/07
Carnival Liberty - 10/07
Adventure of the Seas - 9/08
Ruby Princess - 12/09
Voyager of the Seas - 9/10
Carnival Spirit - 11/10
Oasis of the Seas - 9/13