Water symbolizes knowledge and rebirth in The Miracle Worker, and the play's narration indicates that this is where the "miracle happens" for Helen. Annie has tried to teach Helen to communicate using various objects that are familiar to her: water, doll, cake, milk. Yet in all previous scenes, Helen remains isolated in her own world, unable to decipher what Annie is trying to accomplish with her methods.
Standing at the pump, Helen is finally able to formulate an understanding of all the time Annie has spent grabbing and poking her hands. She realizes that the specific series of touches, varied in their formation, represents a fundamental unit of communication.
It's interesting that water is the first word that Helen recognizes. Water is often associated with a religious baptism; a person dies to an old way of life and arises to a new life. The same is true of Helen. The moment she recognizes that she can spell the word water into Annie's hand to convey a particular meaning, her old life of isolation, frustration, and ignorance is replaced by a life that allows her to form relationships, communicate her needs, and function in her world.
Water therefore symbolizes hope and knowledge in The Miracle Worker, demonstrating the fundamental importance of communication and fulfillment. When Helen is able to spell and to understand the formation of the letters for water, her entire demeanor changes, reflecting a "light" that had not been present before.