US$ always - irrespective of where they are in the world.
Short and sweet the US dollar. A bit longer... on board no currency as everything is charged to your Sea Pass Card which is also you cabin key card and you get this when you embark and you settle up at the end of your cruise. .... Neil.
There is one place you need money and thats the casino. US $ .
And you just get this out against your Sea Pass card too - up to $2000 a day if you wish.