Brant,

I used the TS-E 24mm f/3.5L Tilt-Shift for the last waterfall I shot. I actually liked using the shift feature to make it appear I was closer to the falls than I was (I was further downhill/downstream from the falls). And the shift feature would allow you to capture a wider view if used for a panorama shot (as Jonathan said). The only problem - tilt-shift lenses aren't weather sealed. You'd have to be careful about spray.

Otherwise, the EF 16-35mm f/4L IS USM and the EF 11-24mm f/4L would be decent choices, but....I'd prefer the convenience of the screw on filters if you think 16mm will be wide enough for you. :-)

Sean