The ship is a cashless system. Other than tips for room service or extra tips for other CMs, there is generally no cash utilized onboard. Even those items can be charged. You can use Disney gift cards or your KTTW card at gift shops. When you set-up a cash account, there will be a limit (based on length of cruise) and you will need to return to Guest Services periodically to pay down your onboard account balance before more charges will be accepted.
Enjoy your cruise!