[wojtt]

I would second the choice of 16-35L f4 for a UWA ... nice sharpness and contrast, also dust/water resistant with filer on. That beeing said your 6D might not welcome intensive  water spray if you come really close to the fall.  If I may suggest,  take your tele lens as well, some of Iceland's landscapes really beg for  a 200-300mm  range,  unless compression of the perspectve is totally not your thing    My choices would be then,  a 16-35L f4 and 70-300L  with a fast 20 to 35mm if you plan to go in autumn/winter for the aurora borealis.

 

And yes, I love Iceland  too