Check out sea to summit dry bags.....as light a stuff sacks, and really waterproof. No need for a sealed molding pannier.
I tried the UltraSil ones and found them a bit too flimsy. They were pretty patched up at the end of a 33 day tour. I since moved to the eVac ones (also from Sea to Summit) and think they will be more suitable while still being quite light (the 20 liter one is 3 ounces).
No trouble with either type so far though we keep the ultrasil in the pannier. I strapped an evac to the rack for the great divide and it survived too. I would not try that with the superlight ultrasil.