All living things contain large percentages of water in their bodies. Without water living thing will die. In fact, all living things are created out of water.
"And Allah has created every animal from water: of them there are some that creep on their bellies; some that walk on two legs; and some that walk on four. Allah creates what He wills for verily Allah has power over all things." (Al-Quran; Sura An-Nuur 24:45)
"It is He Who has created man from water: then has He established relationships of lineage and marriage: for thy Lord has power (over all things." (Al-Quran; Sura Al-Furqaan 25:54)
"Do not the Unbelievers see that the heavens and the earth were joined together (as one unit of creation), before we clove them asunder? We made from water every living thing. Will they not then believe?" (Al-Quran; Sura Al-Anbiyaa' 21:30)
Things that are of extreme importance may be taken for granted. What about if the streams, rivers, lakes, seas and oceans all became dry, what would we do?:
"Say: "See ye?- If your stream be some morning lost (in the underground earth), who then can supply you with clear-flowing water?" (Al-Quran; Sura Al-Mulk 67:30)
Or what about the clouds bring rains that are salty:
"See ye the water which ye drink?" (Al-Quran; Sura Al-Waaqi'ah 56:68)
"Do ye bring it down (in rain) from the cloud or do We?" (Al-Quran; Sura Al-Waaqi'ah 56:69)
"Were it Our Will, We could make it salt (and unpalatable): then why do ye not give thanks?" (Al-Quran; Sura Al-Waaqi'ah 56:70)
"And made therein mountains standing firm, lofty (in stature); and provided for you water sweet (and wholesome)?" (Al-Quran; Sura Al-Mursalaat 77:27)
"And He it is Who sends the winds as heralds of glad tidings, going before His mercy, and We send down pure water from the sky,- "(Al-Quran; Sura Al-Furqaan 25:48)
Allah provide us with water that are optimum for the sustenance of life:
"And We send down water from the sky according to (due) measure, and We cause it to soak in the soil; and We certainly are able to drain it off (with ease)." (Al-Quran; Sura Al-Mu'minuun 23:18)
"And We send the fecundating winds, then cause the rain to descend from the sky, therewith providing you with water (in abundance), though ye are not the guardians of its stores." (Al-Quran; Sura Al-Hijr 15:22)
Excess water is designed to drain into rivers and become source of water for places that do not always receive rains.
"And in the earth are tracts (diverse though) neighbouring, and gardens of vines and fields sown with corn, and palm trees - growing out of single roots or otherwise: watered with the same water, yet some of them We make more excellent than others to eat. Behold, verily in these things there are signs for those who understand!" (Al-Quran; Sura Ar-Ra'd 13:4)
"He sends down water from the skies, and the channels flow, each according to its measure: But the torrent bears away to foam that mounts up to the surface. Even so, from that (ore) which they heat in the fire, to make ornaments or utensils therewith, there is a scum likewise. Thus doth Allah (by parables) show forth Truth and Vanity. For the scum disappears like froth cast out; while that which is for the good of mankind remains on the earth. Thus doth Allah set forth parables." (Ar-Ra'd 13:17)
With the rain and water in the rivers, people on this earth can make a living in agriculture, thereby producing diverse plants and animals for food.
"He Who has, made for you the earth like a carpet spread out; has enabled you to go about therein by roads (and channels); and has sent down water from the sky." With it have We produced diverse pairs of plants each separate from the others." (Al-Quran; Sura Taha 20:53)
The rivers and seas are means of transportation and source of foods and ornaments.
"It is He Who has let free the two bodies of flowing water: One palatable and sweet, and the other salt and bitter; yet has He made a barrier between them, a partition that is forbidden to be passed." (Al-Quran; Sura Al-Furqaan 25:53)
"Or, Who has made the earth firm to live in; made rivers in its midst; set thereon mountains immovable; and made a separating bar between the two bodies of flowing water? (can there be another) god besides Allah. Nay, most of them know not." (Al-Quran; Sura An-Naml 27:61)
"Nor are the two bodies of flowing water alike,- the one palatable, sweet, and pleasant to drink, and the other, salt and bitter. Yet from each (kind of water) do ye eat flesh fresh and tender, and ye extract ornaments to wear; and thou seest the ships therein that plough the waves, that ye may seek (thus) of the Bounty of Allah that ye may be grateful." (Al-Quran; Sura Faatir 35:12)
"He has let free the two bodies of flowing water, meeting together." Al-Quran; Sura Ar-Rahmaan 55:19)
"Between them is a Barrier which they do not transgress." (Al-Quran; Sura Ar-Rahmaan 55:20)
All translation of the Quran is taken from Surah.My : Tafseer of Al-Quran in English