Where does the KD boat dock in Cologne? Anywhere near the cathedral/train station?
Out the front door of the cathedral and turn left. Take the first left until it dead-ends at the river. Look for the boats/docks - - probably a bit up-stream. Less than a half mile - - ten minutes?
Hope my information is not out of date, but I recall docking upstream from the cathedral/train station, opposite the Gross St Martin Church. Google maps/satellite shows the location there.