I haven't had to cache water along a trail yet, but if I do, I will use the wine bags. I can't stand the stuff (wine, that is), but I have some friends who drink it, so I do have access to free bags. It might take awhile to stockpile them, but hey, free is free!

I will do as suggested, by hiding my caches off the trail and marking them with my GPSr. I will also write down clues/directions as to where it is and keep that info with me as well.
More adventure in your life today.