"I read that some of the river cruises are free with some of the Rail passes as well."

Only two are with the German Railpass - K-D boats on the Rhine between Mainz and Cologne and on the Mosel between Koblenz and Cochem - none others.

And with a pass you just flash it and hop on. Figure taking a train to the departure point and back to your base if not on the boat route and it could add up to a significant sum - say you base in Cochem and take the train in about an hour to Bingen and take the K-D boats thru the classic part of the Rhine Gorge and onto Koblenz and then train back to Cochem - would probably be 60 euros or so. But like Russ says if you are staying in a town on that classic route then the boat itself may not cost much - depending of course on how long you stay on it.