I would not worry too much about currency exchange. Every place will accept American Dollars easily - along with credit cards.
They do tell you in St. Peterberg to not use dollars, but, vendors on the street did accept them. I paid for a diet coke in the Hermitage with my Visa
I did exchange at the airport upon arrival for some Euro to start. You can also use the Pursers Desk to get small bills. When I was on the Star they had an exchange machine also onboard. Of course a higher rate, but, it's there.
Enjoy your Baltic Cruise - it's awesome.