https://www.k-d.com/fileadmin/KD_Fahrplaene/KD_Fahrplan_2023_web.pdf
Scroll down to page 11 of this 12-page flyer in German to see the schedules for cruise options between Bingen and Koblenz. (The English version should appear sometime in the next several weeks.)
"Fahrplan Nebensaison Rhein" is the shoulder season timetable - April 1-28 + October 4-22.
"Fahrplan Hauptsaison Rhein" covers the high season, April 29 - October 3.
Each of these timetables has a southbound schedule (on the left) and a northbound schedule (on the right.) Traveling northbound is the most time-efficient direction; southbound is much slower because of the current. I've posted an excerpt from the northbound schedule at the bottom of this post.
My comments regarding the Hauptsaison schedule, when most of us will be traveling, follow... they pertain to the northbound cruises...
To cruise the most scenic section, board either in Rüdesheim (which shares the east bank of the Rhine with Wiesbaden) or in Bingen (just opposite Rüdesheim on the west bank of the Rhine) and cruise to either St. Goar on the west bank or St. Goarshausen on the east bank (where the most scenic part ends and most of the boats terminate.)
The first 3 boats of the day terminate in St. Goarshausen. After that, they reverse direction, calling at St. Goar before heading south (so stay on the boat until St. Goar if that is your destination.)
If you want more cruise time, or wish to end the cruise in towns to the north like Boppard, Braubach or Koblenz, you will have some layover time after the first 3 boats; check the schedule for boats leaving St. Goar/St. Goarshausen for points north.
The fourth northbound boat from Rüdesheim/Bingen (16:15/16:30) is different. It travels all the way to Koblenz (arrives 20:00.) This is the Goethe Paddlewheel Steamer and is printed in bold on the schedule. This boat does not operate on Mondays.
Schedule excerpt for northbound cruises from Rüdesheim/Bingen
1) 9:00/9:15 (daily)
2) 11:00/11:15 (daily)
3) 14:15/14:30 (daily)
4) 16:15/16:30* (Goethe, through to Koblenz, Tu - Su only)