Everything is charged to your "account" based on your room card. There is no exchange of cash on board.
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When you check-in at the port, they will ask you for a credit card to setup an onboard account for you. Your stateroom key card is also used to pay for onboard expenses like specialty dining, drinks, gelatos, cakes, shore excursions, onboard shopping etc. At the end of the cruise, all charges that you have will be billed to your credit card.
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