Water is pure: it is what we use to clean things. Water carries away dirt -- and then itself becomes pure again in the course of nature. Much like my footprints might spread dirt, water instead spreads cleanness and purity. Water is how things are renewed. Each Christian is called to baptism, and the water carries much of the meaning: we are washed and cleaned.
Water is one of the absolute necessities of life. The Bible speaks of a deer panting for the water, and of a tree planted by the waters that has no fear of drought. It talks about still waters and restoring the soul. It talks about rain that comes down from heaven and does not return without giving life and renewal to the earth. It talks about a river whose waters make glad the city of God. Time and again, the Word of God uses water to describe God's life-giving role.
The Book of Revelation is a vision, and a book of symbols. But that does not make it meaningless: it makes us need to reach out to see what those symbols mean. We have seen that when the Bible speaks of water, it speaks of cleansing, and renewal, and the source of life -- all worked in us through the Holy Spirit. It reminds us where we are to look for the source of that fountain:
And he showed me a pure river of the water of life, as clear as crystal, proceeding out of the throne of God and of the Lamb. (Revelation 22:1)The life of the world, and the cleansing of the world, flow like a river from the throne of God. That is the river in which we are baptized. That is the still water that restores the soul.