The currency used on board cruise ships depends on which cruise line the ship belongs to! For instance P&O and Fred Olsen will use Pounds, where as RCI and Celebrity will use Dollars. Cunard also use Dollars funnily enough! This is all in theory anyway, since all cruise ships are cashless societies, all bills on board being paid with your cruise card. Just take your credit card to pay your final on board credit bill and you won't need to worry what currency they use! Happy cruising.
On Board What!!
Generally speaking it will be Euros if you are cruising in Europe and US dollars if in the caribbean, however to be certain we'd need to know which cruise line and where the cruise is. Some US ships will use dollars regardless of where they are sailing. I'm assuming that you are a first time cruiser, so apologies if you already know this, but all ships are cashless. Any onboard purchases are charged to your account and you settle the bill at the end of the cruise.