There is another reason to use a credit rather then a debit card. Lets say that your S & S bill is $50 and when you get home they charged your credit card $100 by error. If you cant settle it with carnival directly you can dispute the charge though your credit card. Amex is very good in this respect. for example last month I had a dispute with the Howard Johnson hotel I stayed with in Seattle. Amex withheld the disputed amout from the hotel without charging me while I settled it. You can't do that with a debit card.