You will be given a card to use on board money is payable at end of cruise .The easiest way is to reguester your credit card for this.The card can then be charged in Dollars or your home currency .
The only time you use cash is in the one armed bandits & video games machine. Everything else goes on your card.
You can't use any form of cash on Princess ships. You will get a cruise card on boarding and a final bill at the end of the cruise. One tip, Princess will give you a print out of what you have spent so far at any time during the cruise. It's a good idea to get this to ensure that you are not overcharged as often you only get the final bill during the night before you disemabark.