As of August 2011, Finnish citizens may enter Russia by sea - no visa required. The new marine route will connect Hamina (Finland) and Vyborg (Russia). Interestingly, Russians will still have to obtain a visa to enter Finland.
      

The Namdalingen barge, built in 2000, has only recently been transported to Finland and is currently undergoing a certification. The boat is 27.5m long and carries 139 passengers. The process of obtaining official certificates involves a translation of all on-board signs and documentation into Russian, Finnish, and English languages. The crew is Norwegian.
      

The route will start serving passengers in the second week of August and will be in operation until autumn, or at least until weather permits. The barge will be cruising between the two ports from Tuesday to Sunday every week.
      

Also, as of August 4, 2011 Helsinki is added to the sea route between Sweden, Estonia, and Russia.

Do not forget: you can also travel between Helsinki and St. Petersburg by train.
      

Source: Euromag.