You most definitely can't use a credit card in the shops onboard. I have cruised several times and it was made clear that all charges have to go through your KTTW card. The Disney Cruise website specifically says this, also:
"On board our ships, all transactions are in U.S. dollars. However, you won't need cash while we're at sea because Disney Cruise Line has a convenient "cash-free" system on board the ship. When you arrive at the Walt Disney World® Resort or at the Disney Cruise Line terminal at Port Canaveral, your signature will be requested for check-in. At this time you may also present a credit card (Visa®, MasterCard®, American Express®, Discover® Card, Diners Club®, Japanese Credit Bureau or Disney's Visa® Credit Card) which will give you charging privileges at both the Walt Disney World Resort and aboard the ship. Any purchases on the ship, including beverage services, spa, salon services, photography, medical and laundry services, purchases from our retail shops, gratuities and any item or service of a personal nature must be charged to your room."
I just don't want anyone reading this to be confused about this issue.
"On board our ships, all transactions are in U.S. dollars. However, you won't need cash while we're at sea because Disney Cruise Line has a convenient "cash-free" system on board the ship. When you arrive at the Walt Disney World® Resort or at the Disney Cruise Line terminal at Port Canaveral, your signature will be requested for check-in. At this time you may also present a credit card (Visa®, MasterCard®, American Express®, Discover® Card, Diners Club®, Japanese Credit Bureau or Disney's Visa® Credit Card) which will give you charging privileges at both the Walt Disney World Resort and aboard the ship. Any purchases on the ship, including beverage services, spa, salon services, photography, medical and laundry services, purchases from our retail shops, gratuities and any item or service of a personal nature must be charged to your room."
I just don't want anyone reading this to be confused about this issue.

) of having someone foot our bill, but I did try and surprise DH last September with a purchase in Treasure's Ketch, and was told SORRY!
SO, maybe this has since been done, but definitely not back then when I wanted to do it.