Adding water to wine before drinking was customary, and still is, in many parts of the world. Therefore, this practice comes from the belief that Jesus probably added water to the wine at the Last Supper.
However, there is a lot of meaning in this symbolic action. The Church has always taken the mixed wine and water to refer to the blood and water that came from Christ's pierced side, symbolizing the birth of the Church and the sacraments.
In addition, the wine and water often are seen as symbols of the divine and human nature in Christ. We, humanity, are also represented by the water and because of this, in the past, only the water was blessed.
Now, instead of a separate blessing for the water, this beautiful prayer is said over the chalice containing both: "By the mystery of this water and wine, may we come to share in the divinity of Christ who humbled himself to share in our humanity."
As the few drops of water are absorbed into the much greater quantity of wine, the image is one of our transformation in Christ. The elevation of human nature begun on earth by God's gift of grace will be perfected in heaven by the beatific vision.
An ancient Ethiopian liturgy from the eighth century contains the following prayer to Mary, "As the water and wine in the chalice cannot be separated from each other, so never permit us to be separated from you and your Son."
This is our fervent prayer too.
Most of us probably hardly notice this simple rite of mingling water with the wine which portrays the whole meaning and purpose of our redemption by Christ, that we "share in the divinity of Christ."
There is no doubt that we need to be more attentive to the numerous symbolic gestures that are part of the Mass and learn from each of them as they all have so much meaning.