Why FreshWater? Because Our Body Cannot Store Water!

We simply must drink water and water alone
Source - wrd.org

We all know so well the unique feeling of satisfaction that overwhelms us after drinking a glass that contains only water.
This need to drink 'only water' is a need that is not completely met by the water our body successfully extracts from any other source, even each breath we take or from every food we eat.
Homeostasis - Water as Carrier
Homeostasis is our name for the balance that the human body must maintain (within a fairly narrow range) of many resources - nutrients, minerals and a long list of chemical compounds, including water - for each cell in our body to function as it is supposed to.

Water Homeostasis
Source - bbc.co.uk

Anything we do, even breathe a single breath changes the balance of one or more of these resources in the body.
Restoring the balance by redistributing available supplies of the resource in the body, is the job of our organs and bloodstream.
The transportation vehicle for redistributing the unbalanced resource falls on water.
The water in our body in, thus, simply in continuous motion. Maybe, that's why our body consists (around 65%) of water!
Water Storage in Living Bodies

Camel: Ship of the desert
Source - npr.org

It has been incorrectly assumed that camels have humps because the humps store water supplies. In fact, the humps contain fat and no unusual deposits of just water.
The camel has the ability to drink water in huge amounts (30 liters in 15 minutes, by some estimates) and the ability to increase the water circulating in its bloodstream and the ability to hold water for a longer time than humans can, in their stomachs.

Birds fly long distances without drinking
Source - telegraph.co.uk 

A camel's homeostasis system allows a lot more time, than in a human, to excrete excess water from the camel's body.
Many birds migrate regularly over very large distances and it was assumed that they had a unique ability to reduce their water replenishment need.
Recent research has shown, however, that a bird's need does not change by the distance it travels, but bird's have the ability to literally 'eat' their own organs by extracting the water contained in organs to meet their replenishment need.
Once they get a drink of just water, their organs grow back to a normal size.

Healthy Blood Cells
Source - abpischools.org.uk

Impact or Water Scarcity on Cells

Unhealthy Blood Cells
Source - abpischools.org.uk 

Our bloodstream contains a large amount of water.
If the water in the bloodstream reduces enough, each healthy blood cell starts leeching water to the bloodstream and starts functioning not as the body expects it to. Our health deteriorates.
Occasionally, the unbalance of water in our bodies gets to such an extent that the body prompts the sensation of "thirst" that we can only satisfy by drinking a glass, or more, containing only water.
Relative Approximate Intakes of Water
(Source - http://www.betterhealth.vic.gov.au)

• Infants  0–12 months   – 0.8 liters per day

• children 1–  3 years      – 1.0 liters per day
• children 4–  8 years      – 1.2 liters per day

• girls      9–13 years      – 1.4 liters per day
• girls      14–18 years    – 1.6 liters per day

• boys     9–13 years      – 1.6 liters per day
• boys     14–18 years    – 1.9 liters per day
• women  over 18 years  – 2.1 liters per day
• men      over 18 years  – 2.6 liters per day