By "waterproof", do you mean a drybag-type or just made from waterproof materials?
I have an Exped Cloudburst (25L) that I take kayaking. It's a drybag style but wouldn't be as durable as your traditional (like SeaLine's) boating drybag:
Sea to Summit has their "Ultra-Sil" daypacks. I take mine for layover hikes. It's smaller than the Cloudburst and, imo, the fabric is much more fragile, but it's also much lighter. After only a couple of hikes, mine has tiny abrasion holes on the bottom.