All cruise lines use the cashless system. Your credit card details are taken prior to boarding & your given a seapass card. This card does everything whilst on board. It's your doorkey, payment card for everything on board & your pass card to get on & off ship at each port of call & at the end of cruise you will receive a statement that will be charged to your credit card.
Depending on the shipping line the method of payment can be by credit card, cash or travellers cheque.
What you are unlikely to be able to use as a method of payment is a debit card.
The above generally applies to P and O and RCI.
I just wanted to keep it simple, but your absolutely correct.