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US Dollars is the currency and they only accept Mastercard or Visa,not Maestro/Cirrus.Make sure that you tell them to bill your card in Dollars (by ticking the box on the express check out form you get with your tickets)as your card provider will give you a better exchange rate when the dollars are converted to pounds-don't let Princess do it as this is another money making excercise for them!Use a card as it is much more convenient-the queues at the pursers desk are HUGE on the last couple of days.
All Princess ships use US Dollars. I am confused with regard to your switch credit card. It sounds like a card preloaded with currency rather than a normal credit card where you have a credit limit. If it is a card preloaded with currency then the answer is No. I've copied and pasted a page from the Princess web site which states what they accept. See below -
One of the best things about cruising with Princess is the convenience of being able to sign for all your expenses and have them charged to your stateroom. An itemized statement will be delivered to your stateroom prior to disembarkation. By providing your credit card in advance, you are automatically registered for Express Check-Out. There is no need to visit the Passenger Services/Purser's Desk to settle your account. We accept U.S., Canadian, British and Euro currencies, and traveler's checks. American Express/Optima,® Diners Club/Carte Blanche,® Discover® Network, the JCB Card,® MasterCard® and Visa® are also accepted. Those paying by traveler's checks or cash will be required to leave a cash deposit with the Purser's staff at the beginning of the cruise.
If you are paying by credit card and the card you are using is a Visa or Mastercard issued outside of the US which is not billed to you in US dollars, then the full amount of the onboard account will be converted by Princess Cruises, to the currency of the card at a current exchange rate at the time the onboard account is charged to your credit card. The rate will include a 3% currency conversion service charge. If you do not want your charges to be converted to the currency of your card by Princess Cruises, please ensure you tick the box on the Express Check-Out form which can be found in your cruise documentation.
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Your basic options are to get a standard credit card (note the 3% service charge for Stg conversion), take USD travellers cheques or pay using USD, euro or £ Stg cash.