That's quite a trip! Waterproof packs are difficult to find, but a few companies do make them. One is VauDe, a European gear manufacturer. Its Grand Canyon 65 is a big (4,600 cubic inches), moderately expensive ($395) pack with waterproof fabric, electronically welded seams, and a dry-bag-style top enclosure. Another solution is to buy a waterproof pack liner: Exped makes one that sells for $25 and holds up to 4,800 cubic inches of stuff (www.exped.com). Or, just buy a big dry bag (or a few smaller ones) and stuff your gear into that, then stuff it all into your regular pack. SealLine's Kodiak Sac 25, for instance, holds about 1,480 cubic inches of gear. Cost is $24 (www.cascadedesigns.com).
My own feeling is that a waterproof pack, or a pack with waterproof bags inside, will float. Particularly if you dedicate a single dry bag to serving as a flotation bladder inside the pack. Of course, you'd want to test that proposition before giving it a try under real-world conditions. Otherwise, you could always find a way to attach some sort of flotation device to the pack. Wrap it in a kayaker's PFD, for instance. Anything that can't be punctured, but floats, could be clipped or lashed to the exterior of the pack.
Anyway, hope that helps. Perhaps some readers who have tried the same thing could post their thoughts in the Gear Forum.