Originally Posted by
You want to go where?
It isn't you. It isn't possible to take a cruise from Cologne to Mainz in a day as the current slows you down to much going upstream. You can only do it downstream (Mainz to Cologne). As the best of the cruise is from Koblenz to Mainz, you could take a train to Koblenz and then switch to the cruise.
I like the plan to take a train to Koblenz and then the cruise onward to Mainz. The train takes more or less an hour. How far is the train station from the cuise dock? From Mainz, you can then catch a train to Frankfurt and onward from there with no difficulty. I'd need to check to be sure, but I think the train that goes straight on to Munich makes a stop at FRA, so it would likely be easier to transfer there is you plan to skip Frankfurt.