I carry recycled Tyvek envelopes in my pack. They are waterproof and can be written on with pencil or Sharpie markers.

When one of my suppliers or sales reps. send me something in a Tyvek envelope, I cut it to whatever size I need or leave it intact and still have a tear resistant and waterproof bag/pouch to carry things in plus it doubles as waterproof paper. It doesn't get tossed into the landfill and still serves a practical purpose.

I've cut several sheets down to 4" x 6", punched a hole in the corner and coupled them together with a book ring. Instant waterproof notebook.

I haven't experimented with it yet, but I'm led to believe that you could use Tyvek envelopes as an improvised pouch to carry water if the bottom seam could be sealed to avoid leaks.