In many religions, water is considered to symbolize purity. Water is used in ritual washing - "purification" - in Christianity, Hinduism, Islam, Shinto, Taoism, Judaism, and other religions. Water is also important to the Christian practice of Baptism.
In Judaism, water is used in Mikvah - ritual washing for various purposes. In Sikhism, water is used to make Amrit, which is used in Amrit Sanskar, a baptism-like initiation into the Khalsa. Muslims are required to wash with water before doing ritual prayers or handling the Qur'an in a process called wudu.
Some bodies of water are called sacred in many religions. Among these are the Ganges River in Hinduism and the Well of Zamzam in Islam. In Hinduism, water is also considered to be one of the 'basic five elements' ("panch-tatva"s) along with fire, earth, space and air. The Ancient Greek philosopher Empedocles said that water is one of 'four classical elements' - the others being fire, earth and air. In traditional Chinese philosophy, water is considered one of five elements along with metal, wood, fire and earth.
Muslims believe that "We made from water every living thing." In other words, it is believed that life is made from water (It is a fact that water is essential to life, as you can learn about here). Even as far back as Greek and Roman mythology, Peneus was a river god. There were also three thousand Oceanids: daughters of the Titans Oceanus and Tethys. Each Oceanid was said to watch over their own spring, lake, river, cloud, etc.
As you can see, water is important in many religions. This page only covers a little information about this subject - you can find many useful links and learn more about water and religion on the links page, along with many other subjects.