The Orthodox faith is a sacramental faith. While God might have driven Adam and Eve out of the Garden of Paradise, God still ensured that the living waters issuing from the garden continued to irrigate the whole earth and cleanse its polluted streams and lakes. When we bless water, we acknowledge God's grace and desire to cleanse the world and make it paradise. Water is the blood of creation. Our own bodies are 80 percent water.
Water is also the element of baptism.
By cleansing the water we make it clear again. By expelling the demonic pollutants we ready it for greater service to God. We tend not only the garden that we call nature but also the garden that is ourselves, insofar as we are constituted of water and are born anew by it. We ought not to draw a line that neatly marks off nature from human kind. This is a modern heresy that we have inherited from the Enlightenment.