Although fire is the overriding symbol in this novel, water comes into play several times. One of the most symbolic is the comparison that Faber makes to Montag, trying to convince him to act as a double-agent in the firehouse.
He would be Montag-plus-Faber, fire plus water, and then, one day, after everything had mixed and simmered and worked away in silence, there would be neither fire nor water, but wine. Out of two separate and opposite things, a third.
(Bradbury, Fahrenheit 451, Google Books)
This is symbolic of the new society that Faber -- and the book people, who Montag meets later -- wants to create. The change will come quietly and from underneath, as water at the base of a fire slowly removes the fire's fuel. The Biblical reference is echoed later by Chief Beatty, who remarks, "You think you can walk on water with your books." Montag does not see himself as a Messianic figure, but his passion for books and learning is equally dangerous to the rigid, government-controlled society.
Water also plays into Montag's final shedding of his old life; while he floats down the river, he reflects on the infinite nature of the universe and the way everything is destroyed, eventually. The water gives him the leisure time to actually think his future through and then make the conscious, independent decision to stop burning and start creating.
Faber describes himself as water and Montag as fire, asserting that the merging of the two will produce wine. In the biblical story, Jesus Christ’s transformation of water into wine was one of the miracles that proved his identity and instilled faith in his role as the savior. Montag longs to confirm his own identity through a similar self-transformation.
The references to fire are more complex. In the Christian tradition, fire has several meanings: from the pagan blaze in which the golden calf was made to Moses’ burning bush, it symbolizes both blatant heresy and divine presence. Fire in Fahrenheit 451 also possesses contradictory meanings. At the beginning it is the vehicle of a restrictive society, but Montag turns it upon his oppressor, using it to burn Beatty and win his freedom.