A number of years ago I picked up a waterproof back pack that was made of vinyl with a supposedly waterproof zipper.  It has no pockets. I would guess I have had it (and used it alot) for about 10 years.  I doubt the zipper is totally waterproof, but it does keep stuff dry even in a heavy rain.  I am amazed waterproof backpacks (with zippers) are not more common.  They make so much sense.