I was hoping to get a day pass on the KD Ferry to travel from Koblenz to Bacharach, but it appears to be booked for the day I wanted to travel. Is there any other ferry that I could take to get from Koblenz to Bacharach? Alternatively, could I take a train further to a city further down the Rhine and connect with another ferry line to see some of the Rhine by water? All suggestions are welcome.
Take the train from Koblenz down to Bingen. Get on your ship there and ride it as far as you would like. St. Goar is the preferred end destination, but you could also just go to Bacharach. Trains run up and down both sides of the Rhine all day long.
I have never heard of a KD ship being sold out. Are you using this website?
https://www.k-d.com/en/cruises/scheduled-cruises-on-the-rhine/
I am not aware of a day pass for the KD boats, although with certain rail passes you have free use of the boats for any day you are using the pass. I have never seen a boat booked for the day. Most people just purchase a ticket at the dock.
The prices for each segment of the boat's run are shown on this brochure, last page. Koblnez to Bacharach is 39,80€/P, one way. The schedule is also shown on the brochure. There are only two boats going all the way from Koblenz to Bacharach in a day, leaving Bacharach at 9:00 and 14:00. The trip takes 4½ hrs against the current.
The K-D boats are not ferries. There are ferries that go across the river at Boppard, St. Goar, and Kaub. They carry both people and cars. The K-D boats don't carry cars and they go along the river, not across it. If you go from Koblenz "down the Rhein" (i.e., with the current), you are headed for Cologne, not Bacharach. The Rhein flows northeasterly here.
You are probably looking at the "KDRhine" pass sold online by a travel agency priced at around €32. If it's not available, that's to your advantage. It's expensive overkill for most cruisers. The passes may sell out - I don't know - but the cruise boats (not ferries) are not sold out and do not sell out at all, AFAIK. Once you arrive at the dock, you can surely get tickets on the spot for the day in question.
There are numerous reasons not to cruise from Koblenz > Bacharach. For the most scenic cruise, board in Bingen and travel to St. Goar (only 1.5 hours) as previously suggested (downstream.) Fare is €20 with KD (€16 if you arrive in Bingen on a train ticket and show it at the KD kiosk.)
But it sounds like you want to end up in Bacharach. If so, you can catch a train from St. Goar to Bacharach after the cruise.
Bingen-Rüdesheimer is another cruise outfit that runs boats between Bingen and St. Goar as well; look for "Loreleyfahrten" on p. 1 at the right (in German, English schedule not yet published AFAKL.)
http://www.bingen-ruedesheimer.de/wp-content/uploads/2016/12/BINRUED_Fahrplan2017_FINAL
They charge €18.90 for the same trip. No discount for arriving by train.
The details of your day/stay are unclear but post again if you need help.