There are no cash transactions as far as I saw on our first family cruise two weeks ago. The card is linked to a credit card of your choosing. (You MUST have that credit card with you when you embark for verification.) When you want something, anything, you pull out your Sign and Sail Card, sign for it and the transaction is done! As long as your credit card allows it, all will be charged to your credit card in the end (This is just one payment option) If you want to give your teenagers limits on spending you can state an amount on their card...or none at all.
Each cruiser will have a card and besides for purchases, it is used to get you on and off the ship...at the gate in Cozumel, the border patrol (??) were there to check the Sign and Sail Card of each person passing as only authorized persons are allowed on the pier.
Remember to bring lots of small bills for purchases off the ship, taxi fare, and tips. You're gonna have a a GREAT time!!!!!