All cruise companies are there to remove money from your wallet, so get your currency before you go & use the cashpoints when ashore if you need extra.
do they take pound sterling on board the ship? and is it best to get euros here to spend when ashore?
I have not been on Island so I suggest you ask the question regarding Sterling on the Q&A page, or better still go to Islands Website. As far as buying currency, never buy from the ship as they always charge a Gordon Brown rate (rip off) & personally as I said before, buy your Euros here in the UK, Post Office is one of the best rates, & top up ashore from the cashpoints if you need too.