I’m not sure what to call this: the party barge? The floating cube?  While it’s technically a boat on the river,  I’m not sure it’s properly called a riverboat. Take a look:

Floating Party Cube, Neckar River, Tübingen

That is not an ideal photo of this floating party place, but I think it’s clear enough for you to get an idea of what it looks like.  I took the photo through the window of a restaurant,  which is why there are odd reflections in the upper left (they were on the window).

I have only seen this contraption once or twice on the river.  My initial thought was that it looked like backyard deck someone might have had at their house in the U.S., which  had somehow gotten swept away by the river.  The people would have been standing it on it at the time, and then they just continued to party, since they couldn’t do anything else.

Except, it really is some type of boat, not a backyard deck.  I would assume you can rent it for a party. The people were eating and drinking on it when I saw it; if you click on the photo to enlarge it, you can spot the table at which some folks are sitting. Definitely one of the odder sights on the river here in Tübingen.