Council of Trent
“The holy council in the next place calls to mind that the Church has instructed priests to mix water with the wine that is to be offered in the chalice; because it is believed that Christ the Lord did this, and also because from His side there came blood and water; the memory of this mystery is renewed by this mixture, and since in the Apocalypse of St. John the ‘people’ are called ‘waters,’ the union of the faithful people with Christ their head is represented.”
Source: Council of Trent Session XXII