The information on the Middle Rhine Valley Day Cruise is in German. Unfortunately, I don't speak German. My question is about whether one can begin and end in Koblenz and what is the cost?
Which website are you referring to? KD has an English version of their website as well: http://www.k-d.de/english/home/
The K-D home page, www.k-d.de, has a pulldown link (banner at top) to pages in English (same url as above). This page has links to the schedule and the fare pages. If you want to take a trip beginning and ending in Koblenz, you will have to specify the turnaround point, but the round trip (Hin- und Rückfahrt) will be discounted greatly from twice one way (Einfach Fahrt). I found the boat to be excruciatingly slow, particularly up river (to the SE), against the current. I would suggest taking a regional train as far up the river as you want to go, then coming back on the boat. If you show your train ticket at the K-D ticket kiosk, you should get a discount (up to 20%) on the boat ticket back.
The best section to cruise is between Bingen (at the south end) and Boppard, roughly. If you cruise from Koblenz to Bingen and back, you'll spend 9.5 hours on the boat. The best way to go is to take the train to Bingen and start a northbound cruise there. It's a 2.5 hour cruise to Boppard. Then catch a train from Boppard back to Koblenz. The boat time you save this way allows you to see some villages. A good way to do this is to stop off as you head south on the train to Bingen. St. Goar (Rheinfels Castle) and Bacharach are both worth stopping in, and you can catch another southbound train 1 or 2 hours later (trains run hourly.) The boat schedule is much less flexible for impromptu stops along the way. Then catch the 2:15 or 4:15 northbound boat out of Bingen. Assuming you've chosen to base yourself in Koblenz, I'd also recommend an outing to Braubach for MARKSBURG CASTLE - it's about 12 minutes by train from Koblenz to Braubach.