Swiss 20 Litre Water Bladder. Black.
These are fantastic.
A really robust large capacity water carrier.
Typical Swiss manufacturing quality - with care (or maybe without!) will last for years.
In black rubber so could also be used as a basic shower* - simply leave out in the sunshine for the water to heat sufficiently, then hang in a convenient spot and give yourself a good swill!
Can be stored flat or rolled up ready to use again.
*Note: the water can get quite hot, so additional cold may be required.