It is the purified church. It is a picture of the glorified church made ready to be joined with Jesus forever. Her brilliance was like a very costly stone, as a stone of crystal-clear jasper. However, each has meaning that symbolizes these people of God who have been glorified and brought to dwell with God forever. Just as a bride symbolizes one who has remained faithful and kept herself for the great wedding day, so the church is to keep itself for the coming of the great and final day, etc.
Water gives life, as it restores, refreshes, supports life, etc.
In a sense water is symbolized as life itself and here in the presence of the Lord we have a whole river of it. Here in the presence of God, life is sustained in completeness. Also note John tells of where this river starts, but nowhere does he mention the end, for it has no end. These men looked to the coming of the spiritual kingdom the Christ. However, when man sinned the tree was taken away lest he should live forever in his sinful condition.