WATER

Water is as important as the air (we all know, right? 45-75 percent of our body is water)
Mostly people already know where to get water from and how to treat it.

I will add few methods here which might be  less known.

From the air (basically producing/collecting water vapor) - Air is almost always carrying water vapor. To collect it one would need a surface which is cooler as the surrounding air, to condense the water. In some cases fine meshes (or any other cold material/powder with a big surface) which can catch the water droplets from the saturated air (keep in mind that this water would also contain most of the dust and chemicals from that air too..)

One could create water vapor from the salt water - through evaporation or to get the salts out and apply reverse-osmosis.

Water could be extracted from the ground - mostly on the lower end of the hills and mountains (either collecting directly or through evaporation) Some layers of the earth are not letting the water through and are creating a water source as water is forced to flow along those layers.

Humidity can be extracted rom the surrounding plants by evaporation or juice extraction (caution about juice extraction concerning poison in some plants, )

Every water you can get from the nature has to be decontaminated prior to usage. Last thing you want is to get an infection or irritation, which will cost you multiple times the volumes of water removed from your body, as you consumed before.

decontamination sorted by priority/efficiency:

- heating up over 80 degrees C for few minutes would kill most of the bad bacteria and viruses in the water

- chemical agents like Chlorine (Cl), Iodine (I), potassium permanganate (KMnO4) or Silver (Ag) ions (there are many more existing)
In all cases a correct dosage is crucial as overdosing might cause damage to your tissues or even death.

- filtering through Carbon (Charcoal)
a set of multiple layers of filter media is created in a closed flow-through container (tube, bottle, canister)

First layers (top to down) should be clean sand or tissue, to avoid too much of the dirt reaching the carbon.
last layer should be a clean tissue to make sure charcoal is staying in the container and is not washed out. Filter the water multiple times before usage. Boil after filtering if possible.