The answer is found in the prayer the priest says as he pours the water: “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.”
The water represents us and the wine represents Christ. The mixing of them both symbolizes Christ sharing in our humanity that through His incarnation, passion, death and resurrection, we may share (to a degree) in His divinity. Go, St. Cyprian!
Fr. Vincent Serpa, O.P.