It's normal practice to tip in the currency of the 'ship', but personnaly I use whatever currency I have available at the end of cruise, could even be a mixture!
You could take out the option of what currency by adding it to your cabin account so long as you are happy with the staff you are going to sign off with this option. As you are in the Caribbean the option should be dollars as that is what will be most readily accepted ashore.
If you tip in the currency prevelant in the islands (US $ or Eastern Carribean $) then they do not lose money in exchange rates. However Wilba is right. A mix of the currency you have left topping up with sterling if need be, is fine. Would you turn down free money?
We accept any form of 'lolly' as a tip!