All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Drinking water is permissible, and it is a well-known fact. But if drinking much of it inflicts harm on the body, it would be unlawful, since it is impermissible for one to harm himself or anyone else.
It is proven from the Prophet, sallallaahu ‘alayhi wa sallam, that he said: “There should be no harm nor reciprocal harm.” [Ahmad and Abu Daawood]
One also should not be excessive in drinking water, in response to the following statement of Allaah The Almighty (what means): {and eat and drink, but be not excessive. Indeed, He likes not those who commit excess.}[Quran 7:31]
But if one drinks much of it for a sound purpose, such as seeking treatment or to save his health or so, there is no blame on him.
Allaah Knows best.