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Most cruise companies have the "No cash system" onboard, whereby you give them your card details prior to embarking and you have in it's place a ships card that charges everything to your credit card account, usually in Dollars, but gets converted to the currency of the travellers card by the cruise company and of course they charge for it.
I am not entirely sure what the full meaning of your question is but I will try to give you an answer as I read it.
If you are contemplating paying you final account in money and that money is dollars then that is a good way to pay it because you have already paid the exchange rate for those dollars.
If you are saying should I pay the final account by card and let them do the conversion into pounds sterling then don't, always let your bank do the conversion from dollars as the rate will be better.
The better of these two ways is difficult to say and will really depend on the rate at which you bought your dollars and the conversion rate that exists when the payment is taken from your card and with the rate skipping around as it is at the moment that's anybodys’ guess.
My advice would be to have a Nationwide or a Post Office Credit Card and use that making sure that the ship charge your account in dollars and not in sterling and that you pay that card off as soon as you return to make sure that there are no interest charges because they can be heavy on those cards.
I have friends who have one of these cards and they only use it for holidays and that way they know they are getting the best from that card.
I hope that is not too confusing and that I have got the gist of your question.
I am, of course, assuming that when you leave Las Vegas you will have some money left. Good luck. ... Neil