Thought I'd chime in and reiterate Larry's and others' suggestions.
I often do early season wilderness canoe trips. That means that I'm frequently in some pretty good rain. Couple that with the fact that the water tends to be high from melt, making some phenomenal water to run which is too alluring to portage around and you end up with a pack that is lying down in the bottom of a canoe horizontally, getting poured on and sloshing around in water that was shipped while shooting some rapids.
Nothing is perfect.
I use Nikwax on my packs, inside and out, avoiding straps and pads. I put a contractor bag inside the pack that serves other purposes in camp as well. And I use dry bags. Pack is often hung upside down under a tarp or tent to drip dry.
Works well, with little weight penalty, but some extra packing.