Cruise ships work on a "cashless" system....meaning that when you check in you assign a credit card or a cash down payment to your shipboard account and they use your Key To The World Card (KTTW card) to charge everything you purchase whether it is a drink or a souvie or a spa treatment... it all goines on your KTTW. At any time during the cruise you can add the balance of your Disney Gift Card to your shipboard account to pay it off.
You can't use the physical disney gift card at any point of sale...EVERYTHING goes on your KTTW card.
Your KTTW card is the only thing you need to carry around onboard....you don't need to carry around credit cards or checks or cash or travelers checks or gift cards....you can pay off your onboard account with any of the just mentioned forms of payment....but those can't be used at the actual point of sale.
Your KTTW card is also your ID and room key onboard. When getting on or off the ship anyone over the age of 18 also will need a photo ID with them.
MJ