KD Rhine ferry

"Does anyone know if we can hop-on and off on the same ticket?"

I certainly don't know.

If you are have a rail pass validated for that day, you should be able use it for any number of short trips, but whether a ticket allows you to get on and off at will, the K-D website doesn't say.

But ... it's not like a bus that comes by every 10 minutes. The boat is very slow, and there are only about five boats per day. When you get off, you will be in that town for 1 or 2 hours. Better, pick a part of the Rhein you want to see by boat, say Bacharach to St Goar, do that part by boat and the rest on the train.

And, BTW, DOWN the Rhein is from SE to NW. That's the way the Rhein flows. And the current is very strong. A trip UP the Rhein (NW to SE) takes 50% longer than down the river.

Where are you starting from and where do you want to end?

If coming from Frankfurt, one of the easiest ways is to take the hourly train to Rüdesheim, and get on your ship there. Ride the ship to Sankt Goar and then train back, getting off at the towns you want to visit. The ships are not ideal for hopping on or off. If you want to get to any of the towns on the other side of the river, take one of the ferries that goes across from Sankt Goar to St. Goarshausen.

We are doing the same thing and a couple of people have advised me to do the ferry ride (Bingen to Boppard being the most scenic stretch) and then to use the trains to hop on and off to visit the different towns~ much easier than trying to do on boat.

Looks like you're on the right track.