Just got back and I personally would recommend staying in Warnemunde. We actually took the train to Rostock, but it was just another town to me. The train is very close to the ship. You can use euros to buy a roundtrip ticket and it wasn't too expensive. You do have to make one change of train, but it was easy and took less than 20 minutes. However, I thought Warnemunde was much more interesting. It is actually a seaside resort where many of the Germans go for holiday. It is within walking distance of the port. It has a cute canal that runs along with shops, food vendors, etc. There is also a very long white sandy beach, but the water was too cold for me. We just sat, had a drink, and enjoyed the view along the sea.