All the cruiselines operate a cashless system.
You will be required to register a payment card before your travel.
Everytime you make a purchase onboard, you will hand over a card issued by the ship. Your purchase will be added to your account.
Just prior to the end of the cruise, you will be presented with an itemised bill. Any concerns about the items listed should be raised with the Purser's office.
The bill will be settled against the payment card provided at check-in unless you wish to present the Purser's office with alternative means.