If you go with a private tour company like Red October, Alla or DenRus then you do not need a visa. I am going with Alla this summer and have been most impressed by her speed of responce and the cost is fantastic compared to the cruise ship tours, and you get to see so much more.
You wont need a Visa if you use Red October, you only need a Visa if you want to go ashore on your own (which I believe is not recommended.