I "THINK" a credit card is always required. You do have an option to pay off the balance in cash so you don't end up with the card being charged.
One thing I did notice, and this may have been unique to our cruise....was my bill was split in two and each portion billed to my card seperately. We did the 12-27 New Year's cruise. I got a charged for everything from 12-27 to 12-31 on 12-31, and billed for everything charged 1-1 to 1-3 on 1-3, but they might have done that because of the end of the year coming in the middle of the cruise.