We boarded at Mainz & cruised until we got off to see the castle at St Goar. There was only an hour allowed for this if you wanted to catch the boat back though. We enjoyed the cruise & what we saw but you will need to catch the train back on either side of the river if wanting to get back to your hotel at a decent time. Enjoyed the cruise.
It was raining and there was only 4 of us on the tour which was a double decker bus and it still went so we were very happy about that. The first castle Fredericksborg that you tour was the highlight with extensive things to see throughout each floor rather than the Kronberg Hamlet castle which you only walk around the outside of and is interesting but do have time to do a quick inside run around if you wanted to but enough to interest us in the grounds. We enjoyed the day and definetely thought it was worth it to go on the tour after weighing up the train option which would cost more 6 too hard to get to anyway. We saved money booking via Viator site and pleased we did this.