[rckoepke]

Check out TIZIP SuperSeal zipper -- it was used on some "dry suits" I wear for offshore sailboat racing in cold-water environments and is truly water-proof. It's advertised to be water-proof up to 700 millibar of pressure differential, which is equivalent to 23 feet of water depth. This zipper does require maintenance in the form of regular lubrication using, for example, a food-grade silicone grease/lubricant like the ones used for slushie/daiquiri machines.

I've looked into getting it worked into bags from Montrose Rope and Sail in Scotland when I worked in offshore industrial environments, but they don't have the equipment to do the "vinyl welding" necessary so you'd have to buy the bag without zippers and then find someone else who could do that welding.