Just got off exactly same cruise - we used local ATM to get out about �20 in each country - and what we didn't use we converted back to US$ onboard the ship. Estonia - many places took Euro so no need for Kroon - just ask before you go into shop or cafe. Russia took US$ or Euro or Ruble. We got out �100 worth of ruble at the airport and used that for lunches, shopping and left the rest as the tip for our tour guide.
I suggest, if you have not already done so, you check out
http://boards.cruisecritic.com/ especially the ports of call section on Europe/Northern Europe. This has loads of useful info on what to do in each port. I also hope you have arranged a private tour in Russia and are not going with the ships tours as you will get so much out of being with a small tour group.
if you have any questions I will happily advise what i did in each port etc