Mosel River KD Schedule

I think you are right. Only one boat in each direction per day on the Mosel, and only on Friday, Saturday and Sunday, with a one-stop boat on Monday.
Unless there is another company doing boats on the Mosel.

This part of the Mosel is just OK as far as scenery goes - nothing dramatic like the Rhine - so it isn't anywhere near as popular. KD would schedule more boats if it were.

If you will be visiting Burg Eltz, or Cochem itself, you'd do well to take the train to that area - you can enjoy the river scenery between Koblenz and there more quickly by train. From Koblenz there are good cruise options heading upstream to Beilstein. See the "Schiffsausflug nach Beilstein" schedule at the bottom left of this flyer:

https://www.moselrundfahrten.de/files/moselfahrplan_cochem_2018.pdf

4-5 boats cover this segment daily in each direction. Kolb is the company that operates these boats.

There are other "Schiff" companies running boats on the Mosel.

Thanks, guys! We're coming from Paris on the train on a Thursday morning (Aug. 16th) and wanted to get off somewhere along the Rhine or Mosel, take a river cruise, and then stay (somewhere along one of them) and go see a castle the next morning. Was thinking of taking the train from Paris to Koblenz, taking a boat to Moselkern, and staying at a place I found that's in Pillig and apparently walking distance to Burg Eltz. I'll check out the other schedules, but if the scenery is better on the Rhine, perhaps we'll take the train to Bingen and cruise to Koblenz and take the train to Moselkern(?). Just need to figure out how to get from the train station at Moselkern to Pillig. Thanks so much for your help!

For transport in the area, go to www.vrminfo.de. Click on the British flag to get in English. Use the trip planner to get schedules. A quick look at M-F suggests taking the bus from Koblenz Hbf to Pillig with a change or two.

Was thinking of taking the train from Paris to Koblenz, taking a boat to Moselkern, and staying at a place I found that's in Pillig and apparently walking distance to Burg Eltz.

Pillig is going to involve bus connections. With your short time that will complicate your visit (infrequent buses from Hatzenport.)

...if the scenery is better on the Rhine, perhaps we'll take the train to Bingen and cruise to Koblenz and take the train to Moselkern(?). Just need to figure out how to get from the train station at Moselkern to Pillig.

This is even more complex and troublesome. But YES, you should cruise the Rhine.

"...wanted to get off somewhere along the Rhine or Mosel, take a river cruise, and then stay (somewhere along one of them) and go see a castle the next morning."

Best advice I have is to travel along the Mosel to Koblenz (pleasant Mosel River scenery on the way) and then change trains for Bingen (Bingen Rhein Stadt station.) Walk to the KD dock and board an afternoon northbound cruise boat (14:30? 16:30?) for St. Goar and spend the night there. This cruise shows you the top scenery in the Rhine gorge. And St. Goar's setting is very special:

View across the Rhine from St. Goar

The next morning, you have TWO castle options. Burg Rheinfels (castle ruins with museum, self-guided tour) is right in town - just walk up if you want to go. For a better peek at medieval life, go to Marksburg Castle in Braubach - just ferry across the river and ride the train 20 minutes to the north.

St. Goar ferry

Marksburg is like Eltz in several respects - it's completely intact, never destroyed, and offers a guided 1-hour tour (1 pm and 4 pm in English.) But Marksburg is much easier to get to (walk up from town in about 20 minutes) and it is equally spectacular - love the views over the river from up there. Also, Braubach itself is a real treat and a great place for a meal:

https://www.romantischer-rhein.de/uploads/pics/Altstadt_02.jpg
https://c1.staticflickr.com/5/4143/4751924826_36d7071291_b.jpg

If you want to check out of St. Goar and take bags with you to Braubach, the Braubach TI office will store them for you while you see the castle (when they're open of course, which is pretty much all day on weekdays, reduced hours on weekends.) From Braubach it's easy to catch the train to wherever you're going next. Same from St. Goar.

Cruising the Mosel from Koblenz up river is troublesome as there a lot of lock-and-dam structures along the way that have to be traversed, which really slows things down. Train is the best way, with trains twice an hour and the views out the windows are nice.

Coming from Paris, you can also use the Deutsche Bahn trip planner to pick your route to Pillig. Its a 6 to 7 hour trip. www.bahn.com