Frankfurt river boat sightseeing recommendations

Frankfurt is on the Main River, not the Rhine River. There are no castles in Frankfurt except for a small schloss in the neighborhood of Höchst.

If you want to see castles, you will have to take a train to the Rhine, which takes a bit more than an hour, then get on ship heading north to St. Goar perhaps. Take the train back to Frankfurt. You will see more than 20 castles while on the ship.

Frankfurt am Main is, as its name suggests, on the Main River, not the Rhine, which is not too far away. From the character of your question, is this a long layover or your first day arriving in Germany? You can get a train to Bingen from the airport and take a cruise down the Rhine through the gorge and the home of all the famous castles, but no way is it a 2 hour tour. More like 6 or 8 hours by the time you catch a train back to the airport.

Thanks and thanks for the clarification

A river tour in Frankfurt is worthwhile, but no chance of getting up the Rhine to see castles. imo the eastward tour from the starting point in central Frankfurt is more interesting.

Now, there are, in fact, river cruises of the Main upstream from Frankfurt, but I can't claim to know much about them. The scenery is somewhat similar to the Mittelrhein in parts. Not as many castles, but quite a few scenic towns. I don't know if these tours leave from Frankfurt or some other town further up the river (Seligenstadt or Miltenberg perhaps?).