The first thing we did when we hit each port was go to an ATM (they're never very far) and get a small amount of local currency for incidentals such as local transportation, bottled water, etc.. Most purchases, including food, we did on credit cards (best exchange rates).
The exception to this was St. Petersburg where we were on a Red October tour. Only needed rubles for a few restrooms and our guide took care of that. We only spent money in the Red October store and used a credit card there. (We brought dollars to pay for the tour - there's a 7percent surcharge for paying for the tour with a credit card.)
It's really easy to deal with money on this trip. Don't worry about it.