I'd take something wide. The
Canon EF-S 10-22 USM if you can afford it. If not, the
Tamron 10-24 is good and quite a bit cheaper.
I think you'll need something wider than the 17mm of the lens you have, which on a crop sensor camera gives you the same angle of view as a 27mm lens would on full frame. 10mm will give you the view of a 16mm on full frame, and you may need that for views down in the canyon.
I'd also be sure to take a P&S camera as backup. You wouldn't want to be without a camera and if something breaks when you're down there, it will stay broken until you get out. There are no local repair shops!
The
Canon D10 is probably one of the best choices for a rugged waterproof P&S. It's good to 33ft underwater and drop rated for 4ft drops as well as having optical image stabilization, SD video at 640x480 and a 3x optical zoom.
If you want something a bit cheaper you might want to look at the
Pentax WS80 which is waterproof, has a 5x optical zoom and can shoot 720p HD movies. It's rated for up to two hours in water at depths up to 5ft.