We boarded at Rudesheim (where we had stayed the night before). The boat left at 9:15 AM, and went up the Rhine River to Koblenz arriving a little after 1 pm. We were told there that the return boat to Rudesheim left at 2 pm and would arrive at 8 PM! The ride was 4 hours one way, and then 6 hours back! So, we had to incur the expense of a bus and then a train from Koblenz to Rudesheim. An extra expense that we weren't expecting.
The boat ride was really nice though. Really nice!
I would only suggest that you go part of the way to Koblenz (maybe to Bacharach) and then return from there to your starting point at an earlier hour than 8.