Board the ship. You can't purchase a Coca-Cola card before your cruise.
Wait for the ship to be 12 miles away from shore so you don't have to pay taxes on the card. Locate one of the many bars on your ship. Bring your room key.
Ask the bartender for a soda card. He will ask for your room key so he can charge your account.
Write your name on your card, and use it whenever you want a bartender or waiter to get you a drink. Your bill will be charged to your credit card after the cruise.