Find Your Cruise
The answer given by Alrich is certainly correct, however, depending on which cruise line you are travelling with, many will have either a free or nominal charge shuttle bus. If you are on a Christmas Market themed cruise, I think Hamburg is probably the best coastal Xmas market you will visit.
There is a warehouse renovation scheme going on right by the docks, which has a few restaurants but it is a cab ride to the town centre. Lovely cruise down the river for approx 3 hours to get into Hamburg.