I have no direct experience with that one, but going on what my Avalanche came with, do not expect it to be Water PROOF, like a Fiberglass or Vinyl over the bed rail would be. That cover has three separate pieces with hinges of some type, be it plastic, vinyl, metal, or whatever. The AV has thick rubber surrounded pieces, three of them, removable and lockable; and is about 95% water proof. Notice on that web page they say their cover is water RESISTANT, and not water PROOF, which I think you may get better results in that area with other types, IF that is important to you. I would investigate the other types for water protection.
My AV cover has been much better with water protection though, since I lubed all the rubber with some Vaseline (that Troutly mailed me; he said he had an abundance; )