If you are paying by Credit Card you don't need any cash while on the ship except for tips for the room service servers. Wait, if you have to do some laundry that probably take cash. I don't know if you can get a "cash advance" from guest services from your room card.
Any purchases made on board are done via your room key (Key To The World card of KTTW for short).
Arrival and departure at the port and the use of porters requires cash for tipping them - please do, they work very hard. When leaving, if they haul you bags way out to the hotel shuttle (not the Disney busses) or to your car way out there - please tip them generously - it's their living.
Obviously anything you purchase at any of the stops you will need cash or your credit cards (and you need you photo ID and KTTW card to leave and board the ship at the various ports - INCLUDING
Castaway Cay).
Guest services will cash travelers checks for you.
Repeating the BEST tip in this thread - get your account statement BEFORE the last day to check it over. Don't wait for the final moring statement under the door - you will want to check that too - but if you checked the day before - at your leisure, you will have less to do before disembarkation - which comes way to soon.
2nd Best Tip - You can charge your tips to the Dinning Staff and room steward to your room - It is easy and works very well