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We purchased a small amount of currency before going...we were informed by friends that in Russia that they prefer u.s dollars and that worked fine for us as the ships currency was dollars....in other ports we just used our card.
We did this cruise last year, they took Euros everywhere we went. Our only problem came when we needed to use public toilets, they needed coins in their own currency which we didn't have....very difficult at times. I think the ships carry all currencies so you should be ok.