Zina, re your research about the port of Warnemunde - it's a small seaside resort town with touristy shops along the quay, about a seven minute walk from where the ship docks. Not much to write home about but somewhat picturesque with a lighthouse and fishing boats. Rostock is about a twenty minute train ride from Warnemunde (the train station is between the pier and the quay) and is a small used-to-be East German town whose "city center" is very much alive with families, students (from the university), shops and restaurants and is lovely to stroll through.
On our first trip, we visited Rostock in the morning and walked along the quay in Warnemunde in the afternoon before returning to the ship. On our second trip, we purchased a "transportation only" ship's shore excursion to Berlin (+/- 3 1/2 hours from Warnemunde) which gave us about six hours to explore on our own - not nearly enough time to see all there is to see in Berlin, but certainly enough to get a feel for the city and its people. Years ago we had visited the then "East" Berlin and "West" Berlin prior to the wall coming down and the six hours we spent exploring gave us a great overview of united Berlin. Depending upon how much strength you have when you reach Germany, you might want to consider getting to Berlin. If not, I'd recommend spending time in both Rostock and Warnemunde.