The Rotterdam itinerary parallels the Amsterdam right through Warnemunde, I think. We dock in Warnemunde on 6/30, after which we sail to Arhus, DK, and Oslo before heading to Rotterdam. I believe you sail through the canal after the Berline stop on your way to Amsterdam. We will be in Talinn on 6/24...I've sent a note to their Tourist Office, but haven't heard back. As for Helsinki (6/27) and Stockholm (6/28) we found that they have sightseeing tours (Helsinki has a hop on, hop off bus similar to that which operates in Copenhagen - with head sets offering guide lectures in various languages). In Warnemunde we have reserved a rental car, but would be interested in what you learn from your inquiry with Port Compass! You can email me at firstname.lastname@example.org
if you get something interesting.