As I understand it, your stateroom account is handled in the same manner as when you are charging to a room account at Walt Disney World.
You’ll be asked for a credit card to guarantee charges to your
stateroom account. The card is not billed until you’ve reached the $400 limit for 7 night cruises and $200 limit for shorter voyages (subject to change).
Any On Board Credits will be applied first. Passengers may also provide Disney gift cards and cash towards their room account. Guest Services can provide a print-out of your room charges at any time during your cruise.
Just remember that the Disney Cruise is cashless. You may need some cash for tips such as room service, however most other expenses on board and even on Castaway Cay require using your Key to the World stateroom key card.
On my last cruise, I brought tip money (about 20 one dollar bills) and used a credit card at Check In. However, I was able to apply Disney gift cards to my account by visiting Guest Services. Therefore, the expenses put on my credit card were minimal.