My wife and I were just there in October. I'd advise that you rent the canyoneering shoes and pole from Zion Adventure Co., so you don't break your own hiking poles on the polished bowling balls in the river. If it's early in the season, and early in the day, the dry pants may be an option too -to prevent hypothermia. My wife used them, and I didn't, but I got chilly at some sections.
We hiked up to Orderville Canyon, and the highest water we saw was crotch/waist-height on me, and that section was in the first 10 minutes in the canyon. Other than that, most of what we saw was maybe knee-high in places.
We used dry bags in our packs, but the water didn't get that high anyway. We used our GoLite Rush packs, and they had grommets in the bottom to drain water.
I think that if you're not wading/swimming with the pack on, having grommets is unnecessary; having a dry bag would be fine, and the pack would drain on its own.
Oh, and my avatar is from Canyon Overlook.