In Zion National Park there's an amazing hike known as The Subway. 

The route (http://www.nps.gov/zion/planyourvisit/thesubway.htm) snakes through narrow passages, requires rappells and cold water swims are mandatory. Exped's Torrent series packs are an excellent choice for this kind of trip. It carries solidly, resists the abuse dished out by canyoneering and its roll-top closure is waterproof (not submersible). The Torrent 30 may very well be the best size for this trip; not too big or too small. 

See it here. 

[All photos by Quinton Dowling]