Based on that gigantic load of gear and your mention of water and a shell garment, I strongly reccomend the Mountainsmith Swift or II. I suspect the space will be just about perfect, it's about the size you seem to want, has good enough features to be comfortable... I have one, I like it. I've said enough.

I have been using a Lowe Alpine Mesa Runner for around 10 years. Check it here Carries tons of stuff and is very strong. If you happen to carry it in front it is very stable and does not limit movement.

Burak

This is the waist pack I've switched to recently:

http://www.rei.com/product/14017711.htm?vcat=REI_SSHP_CAMPING_TOC

It holds a lot without being too bulky. I'd like it to be a bit bigger though, so I'm still looking.