I spoke recently with a friend regarding the mixing of the wine with water at the Eucharist. This symbolizes the divinity of Christ and His humanity. Additionally, St. Cyprian compares the mixed cup to the Church being assumed into the life of Christ.
The mixture of water and wine is an ancient custom that even predates the Catholic Church. At the Jewish meals, it was customary to do so.
Water was customarily mixed with wine for drinking in any case, and unmixed wine was reckoned more suitable for washing in than drinking. In the case of the cup of blessing this addition of water was so much the custom that the rabbi Eliezer ben Hyrcanus (c. A.D. 90) reckoned it a positive rule that the Thanksgiving could not be said over it until it had been mixed, though the majority would not be so absolute.(1)And that word "Thanksgiving" is, of course, Eucharistia in Greek. It is evident that not only does the practice extend to the apostlic era, but Christ Himself would have been using a mixed cup at the Lord's Supper.
1) Dix, Gregory "The Shape of the Liturgy" pg 57 (with footnotes)