water

one of the most important things in a survival situation your body is 70 percent water. and you can only live three days with out it.

water pureifucation

but there is a catch to water it may look safe to drink but most of the time its not. and can make you very sick which in a survival situation is like a death sentence. so there are alot of ways  to purify water like boiling it plain you just boil the water :), filtering it you could make one by filling a sock with sand but that wont get all of the contaminates out, they make water filters that get out most containments i would recommend getting one of those,and chemicals that kill or neutralize the containments, like iodine.or bleach