Water has passive as well as active qualities. An example of passivity is that immediately it assumes the shape of the container it is poured in. Its active quality is obvious from the way it dissolves every salt and sweet substance in itself. When solar heat increases, it turns water into vapour and forms clouds. These clouds condense to result in rains. The heat of the sun melts snow and glaciers and raises the river levels. If plants are deprived of moisture, they wither.

If moisture is removed from human body, it turns into a corpse. Three fourths of the human body is water. Only one-fourth consists of dry material. Four-fifths of breast-milk is water. Water is responsible for the digestion of foods and circulation of blood. One can remain alive without food for a long time but cannot survive without water. It is for this reason that every religion has recommended to serve water.
According to the author of Khasais-e-Hissiya, Its reward exceeds the reward of all forms of worship. Specially if water is served in the name of the thirsty Imam (AS), then its reward is much more. Therefore, during the first ten days of Muharram, Sabeels are erected for distributing water in the name of the oppressed Imam (AS).
Hazrat Ali Ibne Abi Taalib (AS) has said, "When the good-doers will be rewarded on the day of Judgment, the beginning will be made with those who served water to a thirsty person.”
(Al Kafi)
In another tradition, Musaadif says he was with Imam Jafar Sadiq (AS) while traveling from Mecca to Medina. All of a sudden, they spotted a man lying under a tree in a terrible condition. Imam Sadiq (AS) told Musaadif, "Come. Let us see why he is in such a state. I think it is due to excessive thirst."
Musaadif says that when they reached near that man. Imam (AS) asked him if he was thirsty. He replied in the affirmative. Imam (AS) ordered Musaadif to give him water at once. Musaadif alighted from his horse and provided him with a drink. Then he mounted his horse and resumed his journey with Imam (AS).
On the way, Musaadif asked Imam (AS), "Master, that man seemed to be a Christian. Is it allowed to serve water even to disbelievers?" Imam (AS) replied, "Yes.”
As stated earlier, it is not possible to survive without water. So what would have been the condition of those who were denied water for three consecutive days? It is a shame that water which is allowed for unbelievers should be withheld from the son of Holy Prophet (s.a.w.a.). Umar Ibne Saad (may Allah curse him) had cut off water supply to Imam Husain (AS), his family and friends at the beginning of hostilities. But when Imam Husain (AS) had wells dug up to provide drinking water to his companions, the news reached Ibne Ziyad. He ordered Umar ibne Saad to be very strict in this regard. He wrote, "O Umar Ibne Saad, you must become a barrier between Husain (AS) and the Euphrates river. They must not get even a drop of it."
(Nafsul Mahmoom by Shaikh Abbas Qummi, pg. 108-109)
Upon receipt of this letter, Umar Ibne Saad despatched 500 horse-men under the command of Umar ibn Hajjaj Zubaidi towards the banks of Euphrates. They alighted at the bank and blocked the water supply. It is well-known that there was no water from the seventh of Muharram. On the day of Aashoora, the intensity of his thirst was such that the world around him seemed smoke. As Jibraeel (AS) had informed Hazrat Adam (AS) "O Adam! Hope you could see how (the oppressed Husain (AS)) will plead due to thirst. He will be able to see only smoke between the earth and the sky."
(Nawaaejul Ahzaan, pg. 265)
The author of Khasais-e-Husainiyyah writes, "Thirst had affected the four organs of Imam's (AS) body. But it does not mean that the rest of his body was unaffected. For thirst weakens the whole body. But the following four organs are affected the most.

(1) Liver - the liver of Imam (AS) had cracked into pieces due to the paucity of water. As he himself says in his call for help "O people! Give me some water to drink. My liver is breaking in pieces due to thirst."
 (Jala-ul-Uyoon, pg. 408)

(2)  Secondly, the lips of Imam (AS) had dried up. In the words of a poet, "My life may be sacrificed on those delicate lips, which had dried up like a flower in winter. And the lips that did not get a drop of water till death."
(3) The third organ to be affected most by thirst was the eyes. There were circles around the eyes. The eyes sank due to thirst. Again in the words of the same poet, "May my life be sacrificed for those sunk eyes which looked towards the Euphrates repeatedly in anticipation but did not receive water till the end."
(4) The fourth organ was the tongue. It had dried up due to thirst. It was completely parched. So much so that he used to chew it every now and then due to which it got wounded. Only Hazrat Ali Akbar could describe the condition of this dry tongue.
What a thirst! Imam Husain (AS) asked for water from Shimr the accursed till the last moment. But when he fell from the horse, wounded, the shameless Shimr moved towards him with the sword and mounted his chest. When he began to slit his throat, Imam (AS) requested, "Even if you are bent upon killing me, at least give me some water to drink.” But the symbol of evil did not pay heed. He killed the Prophet's beloved grandson, thirsty, hungry and oppressed. May Allah curse him till the day of judgment.