The currency on board is sterling. Assume you are going to the Caribbean so would recommend you get some US dollars before you go, which are widely accepted on all the islands. You will probably get a better exchange rate than on board. You only need enough for local spending. If you take any of the ships excursions, these will be charged in sterling but local taxi drivers use US dollars.
Cruise ships are a business geared to taking money from you. Currency Exchange is no exception.
Also going on the 16th (might see you there!)
We have been to the region 4 times and always take $US which are happily accepted everywhere
If you pay in US make sure you get your change in US -the local currency is not much use to you
On the ship everything is charged in sterling to your sea pass and you get a bill at the end
I am assuming the on board desk will let me have pounds via my debit card for tips as I do not want to carry large amounts of cash.