Is there any river boats that run down the Rhine in the winter? (going in December) wanted to go to the Marksburg castle and then take the Rhine down from there... anyone know if it stops completely in october?
There are cruise lines that run all year long, but I don't think they make stops all along the river. Most base out of Koblenz.
This thread had some information about winter cruises on the Rhine.
https://community.ricksteves.com/travel-forum/germany/does-anyone-cruise-the-rhine-in-november
"... wanted to go to the Marksburg castle and then take the Rhine down from there... "
Boat travel would be much worse than chilly in December. Local trains run hourly or better along both sides of the Rhine, stopping in almost all the towns, and offer good views from the windows in addition to warm seats. Take the train to Braubach for Marksburg Castle. Going downstream? Head north to Koblenz, the next big town. Going upstream? Head south from Braubach along the east bank. Once you hit Rüdesheim, you've seen the most scenic section of the river. Wiesbaden is the next big town after that.
From Braubach, it is also possible to visit the more popular west bank villages (Boppard, St. Goar, Bacharach, Oberwesel, and Bingen, the end of the scenic part) if you first take the train to Koblenz and change there. Mainz lies further south.
I should also note that I believe Marksburg only offers one or two English tours a day during the winter. Unless you're comfortable with a German tour, you may have difficulty properly timing a boat excursion to the English guide.