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It is US$.
Very little of hard currency is necessary as you join a cashless society and everything is signed off against your seapass you will all get one when you check-in. Normally the lead person has to agree or otherwise if any other member can spend using their seapass this is an individual decision, particularly if teenagers are part of your party. Only in the casino might you need hard currency and of course when ashore.
What currency is used ob n.c.l. cruise ships in the casino on mediterranean cruises.