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On most crusies you don't need cash. Anything you spend on board is charged to your account which you can then settle in cash, or by debit or credit card at the end. You will want some dollars when you go ashore or on excursions of if you are in the US pre or post the cruise.
You don't need any actual cash to use on the ship itself as it is cash free. An account will be opened for you on the ship and you will be issued with a card to use for any purchases which will be added to your account as well as any excursions, beauty treatments, etc. then at the end of your cruise you settle the account either by credit card or cash. If you use your credit card remember to tick the appropriate box on the form you fill in with details of your card, saying that you want the total to be left in dollars and your credit card will convert it back to sterling. You get a much better exchange rate this way.
So you will probably only need cash for when you are ashore, not sure what currency you will need as I haven't been to the caribbean yet!...presume it will be dollars but am sure someone else will help you there.
You do not use cash onboard. Everything is charged to your cruisecard and a single payment made at the end of your cruise on your credit card, or you can pay cash if you want. You will need US dollar cash for spending on the islands off the ship. Dollars seem to be widely accepted everwhere and saves changing into local currency.
Dollars are the best currency for taking to the Caribbean for spending ashore and also if you intend to give tips to staff in cash rather than through your on board account. You will not however need any form of cash fro paying for anything onboard as that is always debited to your onboard account by you presenting your cruise card easch time you want to pay for something. Not surwe what all the options for settling your account are with Princess, but usually cruise lines will accept either a debit or credit acrd. Some will accept cash too but usually expect you to lodge a deposit at embarkation. Check the princess website to find out what options they give you.
IMPORTANT: Princess cruises calculate your on board account in dollars...if you are from UK they will then convert to uk pounds at a very poor rate AND then they will charge you 3% for doing so. In your pre cruise documentation you will get a form and in the small print there is an option to tick "do not convert", which should prevent this happening. It is still worthwhile checking on board that this option is on your account. Also you could settle your account in dollars at the end of the cruise.
Many thanks for infofmation ken Skingsle.