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You will probably find the conversion rate used will vary day to day.
There have been some recent posts on how to minimise the cost of concversion. Look up post of the lats week or so for the information.
The Boat doesn't charge any percentage for dollars as you can only pay in dollars if that is 'currency' on board, if paying by cash. If you pay by credit card, as most do, you let your credit card company make the conversion charge.
It is better to sort our your currency before you go as you will get far better deals here than on the ship. Some take euro or dollars travellers cheques which you can cash on the ship for a 100% deal, depends if you are cruisng in Europe or the Americas or Caribbean areas as to whether it is euros or dollars and of course you can pay be credit card but don't let the ship do the conversion to pounds but your credit card company.