If one were to, say, be considering the RS tour next summer that ends in St. Petersburg, what are the non-flying options for getting to Stockholm at the end of the tour? Is there a ferry? I have found conflicting reports on this site and on Trip Advisor, so up-to-date info would be much appreciated. Thanks!
Since St Petersburg Ferry to Stockholm Ferry does not sail everyday and appears to rotate to different days another alternate would be to take the speed train from St. Petersburg to Helsinki which has Ferry sailings to Stockholm everyday. In any case it is a long sail. I did it on a night Ferry long ago.
Check on the St Peter Line which has a route that connects St Petersburg, Helsinki, Stockholm, and Tallinn. I've never taken this line, but have looked into it as I want to visit St Petersburg on a future trip. You might also consider starting your trip in Stockholm and then take the overnight ferry (Tallink/Silja or Viking) to Tallinn.
You might want to run over to the RS Headquarters in Edmonds and talk to their Travel Desk or even a Travel Advisor. TA costs a little money but if you sign up for one of their tours, you get a 50% discount in a TA session.
We take a RS tour every summer and always take advantage of that discount to get added information related to our own itinerary.