water purification tablets
And there's another! Why don't these people think of these things? It's like they apply to appear on the show, but they're never watched it. So it's like they're oblivious to the same kinds of problems each pair runs into.
Maybe it's that if you've got the guts and confidence to volunteer for something like that, you think you don't need to learn anything from prior contestants/victims. Strangely, no one dropped out voluntarily on N & A's first season, and they didn't have any way of knowing what it would be like from watching a previous season. Later seasons find people going in all cocky, and then tapping out, and they have the benefit of knowing in advance what they'll be in for! (And some of these people wussing out are survival instructors; to fail at what you claim you're good at, on national TV, can't be too good for that part of your career!)
I kinda glossed over the plastic sheet thing, but that's brilliant too. All the people on these shows are actually amazingly good at making shelters, but they can leak a little in a downpour. Would make a good inner liner, from the rain as well as wind. And bugs, if it was big enough. Which leads me to the last puzzling thing I'll mention; in some locales the bugs eat them alive, and in some they freeze their butts off at nights, and I can't help but wonder, how come no one has ever gone subsurface with their shelter. Some, esp. in jungle areas, have made very cool raised shelters, I guess because of army ants and venomous snakes. But there's nothing insulating like earth.
(p.s. You might also be thinking using the plastic sheet to condense steam. One guy on Dude You're Screwed had a plastic baggie, and put some juicy leaves in it and left it in the sun, and got a bit of fresh water that way.)