I have been to Rostock. It is an interesting typical German town, but I am not sure it is worth the visit from Warnemunde (where the cruise ships dock) which is a delightful little beach town - the main seacoast town for the denizens of Berlin.
There is a regular train between Warnemunde and Berlin, and one can see a fair amount of this world famous city in just one day.
I am the editor, but I also speculate, ask questions and play devil's advocate. I reserve the right to change my mind.