I don't know about waterproof packs but my DH puts the cameras, tapes, cards and accessories into separate Zip lock baggies and then puts tehm into the camera bag to keep them from getting wet. He doesn't care about the camera bag getting wet.
You can buy Scotch Guard spray and coat your bag with it. All it is is a waterproofer. It shouldn't ruin your bag. Test a small spot first but it works great.