To open your safe you have to have a card with a magnetic strip. But they advise you not to use your sail and sign card. But you can use any card with a magnetic strip, but you have to use the same one you use to open it to reopen it.
Not all ships use the magnetic strip for the safe. We were on the Fantasy and you program in a code to use for the trip. However I am sure you could just take one of those grocery store cards to use if you don't want to use a credit card.
We used one of the gift cards from Walmart. Find a convenient location in the cabin to hide this card, and it will be available for use quickly. This way the card will be handy to use and you won't have to remember which one was used last.