Often in the Bible, water functions as a symbol of chaos and sin: the waters at the beginning of creation, the waters of the Red Sea, the waters of Noah's flood, etc. Just as the Spirit of God hovered over the abyss in the beginning, so the Son of God walks on the waves. This signals God's lordship over all of the forces of destruction that confront us. As long as we look to Jesus, we can walk on those same waters with him.