Don't have any info on the Rhine.

But we just spent a month in Germany and the Czech Republic. We took the
following short cruises:

Hamburg on the Elbe and associated city channels.

Heidelberg to Neckarsteinach on the Neckar, with no problem.

Trier for two hours on the Moselle, with no problem. This was our most
enjoyable cruise of the trip. The vineyards along the banks were beautiful.

Kelheim to Weltenburg on the Donau (Danube), we were unable to take because
of low water. So we drove down just to see the old Kloster brewery once
again, and have a good meal there.

Gute Reise,

Joe in Texas