Thanks for all the replies so far.
@GvG: I am aware of the fact that Helsinki is not one of the Baltic states.
However, itís a port very close to the Baltics and maybe the only option for us to get to Tallinn is via Helsinki (do you know of any other connections?). Reasonable prices pop up when there is some competition. My friend Google has so far only found Finnlines operating in that area. The ticket is too much for our budget. Thatís why I am hereÖ
@Pecha72: thanks for sharing your experiences. Ideally we would like to go from Poland (somewhere) to Tallinn and then via the Baltics all the way down to Germany. As I mentioned in the first posting it seems that Rostock-Tallinn is not available anymore (but please do correct me if I am wrong).