The ticket says you need to print it out - we bought ours already in Dusseldorf, because it was raining in our last day there, and it seemed like a good option. Thankfully, our driver wrote down the booking number and let us go either way (we went on a very slow day for turism, i don't know if this would hold up for all drivers and all time slots). Nevertheless we did waste an hour searching for somewhere to print it out and it was not nice. It makes no sense to print out a ticket which presents a QR code in 2019. The trip itself was very nice, and the audio gave us some interesting nuggets of information. It was raining, and it was a good chance to see the city while getting out of the rain and prevent getting sick. And we wouldn't know those facts otherwise. It even cleared up some stuff we had seen the day before. We didn't manage to finish the entire thing, because of the time we wasted on the printing issue, and we ran out of free-time.

The cruise boat staff were very welcoming and we got to see the main sights along the Rhine around Dusseldorf old town and newer aspects of the harbour such as the Frank Gehry building on the one hour panorama cruise. The company offers light snacks and drinks on board and the waitress service at your chosen table was very good. The apple strudel was delicious! The boat itself was very clean, nicely decorated and furnished with tables and chairs rather than benches to sit on. The narration was in German and English. I would certainly recommend a KD cruise as an introductory way to explore the city.