Lenses Needed for Rhine River Cruise?

In a few weeks I will be taking a river cruise on the Rhine River
between Basel, Switzerland and Antwerp, Belgium with a side trip on
the Mosel River.  I have visions of sitting on deck leisurely taking
photos of castles and the like as we drift by.  My question is given
the width of the rivers, what lenses do I have to take to do this.  My
basic kit includes in long lenses a Nikon 80-200/2.8 AFS with Kenko
Pro 1.4x and 2x extenders.  But I can also take my Sigma 50-500 AFS,
which can also produce good results with the 1.4X even at 700mm. 
However, given its weight and all the other photo equipment, I would
prefer not to unless really necessary.

For those who address the photo aspects, can you give me some idea how
wide the rivers are at various points?

Thanks in advance for any advice that can be provided.