If you rather not use a credit card, you just go the "Guest Relations Desk" and give them the amount of cash you think you'll spend. If you spend less, you'll get cash back. If you spend more, your sea pass card won't let you purchase anything until you go to back and put more into your account. Your sea pass card is your key to your stateroom, and your charge card for purchasing anything on board the ship. While in your stateroom you can check your balance on your tv by selecting it from the menu, or you can go the guess relations deck and inquire.
Hope this helps,