From RCI:
Q: How do I pay for onboard purchases?
A: All of Royal Caribbean International ships operate on a "cashless" system, meaning your boarding card, (also known as your SeaPass card), will be used as a charge card to make all onboard purchases. Normally to activate this SeaPass® account, most guests provide a credit card, (American Express, Optima, MasterCard, Visa, Discover and Diner's Club), to have these purchases charged to at the end of their cruise. Additionally, the SeaPass system may be activated with traveler's checks, debit cards with a Visa or MasterCard logo, or cash.
A running tab of all your purchases will be kept under separate folio numbers for each guest and an itemized statement left in your stateroom the night before disembarkation. If you provided a credit card and there are no discrepancies, the amount will be charged to your account on the morning of disembarkation. If you have used traveler's checks or cash to activate the account, you will need to settle the account at the Guest Relations desk, (to either receive a reimbursement of the unused amount you originally left or to pay the balance of what you have charged over the amount of what you originally left).
Please note:
We require guests to place a deposit if they are paying for their SeaPass account with cash. There is a $500 daily limit on cash accounts for 7+ night sailings and $300 for 2/3/4/5 night sailings. Once the daily cash limit is reached, we will call the guest and inform the guest that the limit has been reached.