This is what it says on the P&O website "For your convenience, P&O Cruises operates a cash-free system on board with Sterling as the currency. This is an automatic process; all you have to do is register your credit card during check-in at the beginning of your cruise. No further action is then necessary. You sign for all purchases and simply settle your account by credit card or debit card at the end of your cruise. We accept Visa, Mastercard, American Express, Diners Club, Visa Delta and Visa Debit. We regret we are unable to accept Switch/Maestro, Solo cards, American Express Prepaid Travellers Cheque Credit cards or Visa Electron. We are also unable to accept payment via cheque or by cash." In reality I have heard of people paying by cash even without a credit card and I am sure that you can still pay off part or all of your account in cash.