Our ship stopped within walking distance of the train to Rostock and it is easy to do. We needed marks then, but now it is euro. I think we put it in a machine and got the tickets.
This should be able to be done on your own. You just need to know what you will do so you don't waste time deciding what to do when you get ther.
Get an early train back. Always carry your passport in the unlikely event you miss the ship. You can catch it at the next port.