Frankfurt to Koblenz

If it was me and you only have one night, go ahead and take the train to Rüdesheim and from there the ship up to Koblenz. You see the most castles that way. It should be really pretty in April. Alternative is to get off the ship at St. Goar, explore Burg Rhinefels and then finish your trip by train to Koblenz, or you can train to some other towns, like Bacharach.

Take the train to Rüdesheim that leaves every hour from track 23/24 from the Frankfurt Hauptbahnhof. Schedule and prices can be found here:
www.rmv.de

Ship times you can find on the KD website. Use your train ticket to get a discount on your ship ticket. The ship leaves Rüdesheim at 09:15 so you will need to get to Rüdesheim rather early. If you need more time, get on the ship at Assmanshausen as it stops there at 09:45. At this time of year, there is only one ship a day, so don't plan on using this as a hop on hop off means of sightseeing.
https://www.k-d.com/en/home/

There are several KD boats per day beginning April 3: KD SCHEDULE
But hopping off and on with luggage means sightseeing with luggage in hand (there are no station lockers in the Rhine towns) unless the TI office in Town X is open and will hold yours. So you probably should stay on the boat for the duration of the cruise.

And because the Rhine towns are MORE interesting on foot than from the boat deck, you should probably get off the boat after you're through the most scenic part (which would be St. Goar,) drop your bags at a hotel there, and spend the night in this most scenic spot instead of Koblenz. This solves the luggage issue and allows time in some very nice old-world places.

View across Rhine from St. Goar

Your Rüdesheim - St. Goar cruise takes about 1.7 hours. There would be time to see Rheinfels Castle and Bacharach (10 train minutes south of St. Goar,) as Jo suggests. Boppard (10 train minutes north of St. Goar) is also very nice.

Alternatively, stay on the boat a little longer to Boppard (2.5 hours) and stay there. It's a larger and also very attractive town with a little more to do in the evenings. You can train back south to visit St. Goar (10 min.) and Bacharach (20 min.) for sightseeing.
Boppard's market square
Boppard street
View over Rhine and Boppard from cafe near chairlift

But I don't recommend cruising through the waning scenery to Koblenz (which is only just OK) or staying there overnight when these handsome towns are just to the south. The next day you'll have a 15-minute train ride from Boppard to Koblenz (10 min. longer from St. Goar) where I guess you plan to move on to somewhere else.