One more thing,
Do not worry about needing a visa if you use a local tour company in St. Petersburg. Getting through customs there was a breeze as long as you ignore the sourpuss agents!
All local tour companies will provide you with a tour ticket(via email that you print and bring with you) that has their official company stamp on it, so then all you need is your passport and the immigration card that your ship gives you in your stateroom.
I found it easier to go through customs in St. Petersburg than going through airport security!
Don't believe any scare tactics or intimidations by the cruise lines that you can't see St.Petersburg unless you book a ship tour. Total lie and bogus.