Books and the places where they are hidden are eradicated by fire - they are burned out of existence so as not to contaminate society, just as dirt is washed from one's hands. Captain Beatty mentions the standard practice of immediately cremating the dead so as not to be burdened with decaying bodies and memorials and the grief associated with them. Fire also represents awareness. It smolders for days. Well, this fire'll last me the rest of my life.
Fire is important for it's transforming powers as well.
In the opening paragraph of the novel, the author refers to the pleasure Montag took in seeing things changed by fire. Similarly, Montag is changing with each fire he sets. Water, the opposing force to fire, takes on meaning as a metaphor for escape. Every night the waves came in and bore her off on their great tides of sound, floating her, wide-eyed, toward morning. There is also an image of Montag finding escape through water, though he is escaping not from himself, but from the mental hold of his oppressive society.