A few water-carrier generations ago, circa 1985, Adventure-16 made a 2-gallon water carrier consisting of a light plastic bladder with the same filler/spout as wine-in-a-box fit inside a slightly smaller nylon hang sack (hence the plastic liner never took any force, only the nylon did). I saw that and immediately thought, "Grand Canyon in Summer". Hike down hill with it deflated (or nearly so), but fill it up for the return trip. Yeah, water is heavy, but this way it wasn't poking you in the back. Empty it was 1.5-2 ounces. While backpacking, it was lovely for:
1) carrying 2 gallons from the lake or stream back to camp, hung from a tree, it was a convenient water source all night and the next morning.
2) they made (and I've MYOGed since) a mini shower head. It was (still is, maybe) the lightest, highest-capacity shower you could have.
3) inflate it with air, wrap a t-shirt around, and you have a pillow for use at night.
4) as you depleted your food and lacked the volume to fill your pack, the water carrier could be inflated with air and placed at the bottom of the pack to keep weight higher on your back.
I still use mine at times.
Especially in GCNP, I've imagined it could be handy in an emergency - someone suffering heat stroke, say. You can't reasonably carry them to the nearest water source to cool them down, but a runner with that water carrier could bring 2 gallons to the victim. Also, when dousing a campfire, you don't scrimp on the water if you can carry two gallons in one trip.
You can MYOG your own with a 5-liter wine-in-box bladder (dumpster dive behind the low-rent apartments and you'll always find one) placed in a slightly undersized stuff sack. Hot-knife a hole for the spout/fill spigot and stitch a hang strap on it.
In a pinch, remember your dry bag not only keeps water OUT but it can keep water IN. If opposing trail traffic tells you about an unexpected 10-mile waterless stretch, or you in some first-aid emergency, that dry bag becomes a 5-, 10- to 20-liter water carrier.