It is possible but it is a long way and if you hire a car or the train is delayed and you miss the sailing time the situation becomes untenable. So I would suggest on this occasion to forget it and stay local, or take the official tour. Neil Dow.
Go to boards dot cruisecritic dot com and then scroll down to the ports of call section and then click on the link for Europe - there will be lots of info in there on how to DIY.
We did a Baltic Cruise with Celebrity in May and did a trip from the port Rostock to Berlin which is a 3 hour train journey the ship hired the train which was full of people going on different trips,on arrival in Berlin we then boarded the waiting coaches for the different tours,we then all met up at a certain time at the train station for our journey back to the ship,it was a long day 12 hours altogether but worth it Regards Iren.