This may not be what you're looking for:
I use military surplus packs of different sorts. Some have more than one pocket, others do not. Even the plain canvas ones qualify as water resistant, and some of the packs are actually waterproof--I have some East German combat packs which will hold water. The advantages of using this sort of thing are the following: They're usually under $10.00. They're very sturdy, better than most bicycle specific panniers, I would guess. They're also useful as backpacks.
I usually use toe-clip straps to attach them. The backpack straps can either be tied up or in some cases removed.

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