Cochem, train, auto and River cruises

Cochem is on the Moselle, which joins the Rhine at the Deutsche Eck in Koblenz. That should be your point of reference. Most people start the Rhine cruise (which goes to St Goar) from Rudesheim/Bingen and cruise downstream (North) because that's faster. To cruise downstream to Cochem you start in Trier. Going upstream (either river) is pretty much an entire days trip (from Koblenz).

You can reach Cochem by train from Trier, Bingen, or Koblenz (or any place along those routes) easily. So one of those cities should be your "overnight". If you have 3 days I recommend basing out of either Trier or Koblenz. Koblenz because you can see the Eck, the the Fortress Ehrenbreitstein, and travel both the Rhine and Moselle by boat or train easily. And it's a nice city. Trier because it's got fantastic Roman ruins (it was the biggest city in the Empire outside Rome until ad. 500), it's a prettier city, and you can easily take a short train ride to Luxembourg or Saarbrucken, or even to Metz for something very different. If you want to be on the Rhine (further South) stay in Mainz, which I like very much also.

If you have a car all of this is moot, as everywhere I have mentioned is within a 2 hours drive radius.

If you're going this weekend remember there's a wine fest on the riverbank Friday - Sunday. Bring a raincoat, the weather has sucked.

The Mosel River starts in France and flows north into Germany and into the Rhine River at Koblenz. Taking the KD boat to Cochem would be slow going as it is against the flow and there are several locks along the way. Train would be the ideal way to get around, There are day tickets offered by the VRM (the district transit authority) that give unlimited travel for a day or multiple days on all the trains and buses in the district, which includes St Goar and Cochem.
https://www.vrminfo.de/

There are day cruise boats operated by Kolb Brothers for tours from Cochem.
https://www.moselrundfahrten.de/

If there is a week end wine festival in one of the villages during your visit, don't miss it.

If I take the KD boat I it seems a lot of people get off at St. Goar.

That's partly because 1) most K-D boats terminate there, and partly because 2) the best Rhine scenery lies just to the south, between St. Goar and Bingen. Between Bingen and St Goar there are 4 boats per day on most days that make this trip.

Please note that K-D boats are SIGHTSEEING boats first. Using them for TRANSPORTATION is possible - but usually inconvenient.

I won't be able to make it to make the KD boat up the Rhine river in time

It sounds to me like you might be trying to transport yourself by cruise boat from MAINZ (where there is only one boat per day!!) all the way to Koblenz. If so, please drop this idea. It's way long and way expensive as well as way too early in the day for most people, and it's not all that scenic if you start there.

...but how do I get to Cochem from there?

Suggest this plan for reaching Cochem...

• Train (from wherever you start that day... where is that???) to Bingen Rhine Stadt station (or Rüdesheim.)

• K-D boat rom Bingen (or Rüdesheim) to St Goar (or maybe further to Boppard if you have the time; at most times of the day a separate K-D boat takes you from St Goar to Boppard if you wish, more €, of course.)

The map below shows the railway that will take you from St Goar (or Boppard) to Koblenz... and other stuff too.

http://www.loreley-info.com/eng/rhein-rhine/walking-hiking.php

• Train from St Goar > Koblenz > Cochem (change trains in Koblenz.)

The rail map below shows you the Koblenz > Cochem > Trier train trip.
https://i1.wp.com/cityscape-travel.com/wp-content/uploads/2020/10/RPBahn2.png?fit=1024%2C1171&ssl=1

Your current plan does skip some of the nice places on the Rhine River. But if the Mosel is your focus, there is nothing wrong at all with staying in Cochem. It is a lovely and popular town with lots of hotels, a hostel, apartments etc. and a very good travel base for a train outing to Trier, Traben-Trarbach, and other nearby towns. Bernkastel (maybe the most beautiful of Mosel towns) is not on the train line; take the train to Wittlich and the bus connection from there to Bernkastel's Kues Forum stop. The train scenery between Koblenz and Traben-Trarbach follows the river and is wonderful, among the best train rides in Germany.

Cochem is also an excellent base for getting to Burg Eltz Castle, Rick's favorite castle in Europe, I believe.

But you can also take a river cruise. From Cochem to Beilstein is a lovely 1-hr. cruise. From Traben-Trarbach to Bernkastel is also nice, maybe 2+ hours one way. (I would not plan on cruising Cochem >< Koblenz. K-D has dropped service on this route. Use the train here.)

If you enjoy hiking or biking, the Mosel is the place for that without question.

Rick's Mosel information is somewhat limited. Here's a page of introduction to several nice Mosel destinations:

https://www.mosel-inside.de/en/travel-guide/mosel-villages.html

There's some really pretty countryside north and west of Cochem, away from the river, which is more easily accessible by car; you only have 3 nights in Cochem.

The Mosel's my favourite area outside of the alps region. A great area for hiking, if you are interested, easy to difficult levels, including the steepest vineyard around (Calmont).

Cruise to Zell, which is worth a stop for lunch. Beilstein, closer to Cochem, is a lovely little spot. Burg Eltz just up the road. A longer day trip, but more than worth it, to Bernkastel-Kues, which just edges out Cochem for me (think you need a train and bus combo).