Objection 1. It seems that the mixing with water is essential to this sacrament. Because Cyprian says to Cecilius (Ep. lxiii): "Thus the Lord's chalice is not water only and wine only, but both must be mixed together: in the same way as neither the Lord's body be of flour only, except both," i.e. the flour and the water "be united as one." But the admixture of water with the flour is necessary for this sacrament. Consequently, for the like reason, so is the mixing of water with the wine.
Objection 2. Further, at our Lord's Passion, of which this is the memorial, water as well as blood flowed from His side. But wine, which is the sacrament of the blood, is necessary for this sacrament. For the same reason, therefore, so is water.
Objection 3. Further, if water were not essential to this sacrament, it would not matter in the least what kind of water was used; and so water distilled from roses, or any other kind might be employed; which is contrary to the usage of the Church. Consequently water is essential to this sacrament.
On the contrary, Cyprian says (Ep. lxiii): "If any of our predecessors, out of ignorance or simplicity, has not kept this usage," i.e. of mixing water with the wine, "one may pardon his simplicity"; which would not be the case if water were essential to the sacrament, as the wine or the bread. Therefore the mingling of water with the wine is not essential to the sacrament.
I answer that, Judgment concerning a sign is to be drawn from the thing signified. Now the adding of water to the wine is for the purpose of signifying the sharing of this sacrament by the faithful, in this respect that by the mixing of the water with the wine is signified the union of the people with Christ, as stated (6). Moreover, the flowing of water from the side of Christ hanging on the cross refers to the same, because by the water is denoted the cleansing from sins, which was the effect of Christ's Passion. Now it was observed above (73, 1, ad 3), that this sacrament is completed in the consecration of the matter: while the usage of the faithful is not essential to the sacrament, but only a consequence thereof. Consequently, then, the adding of water is not essential to the sacrament.
Reply to Objection 1. Cyprian's expression is to be taken in the same sense in which we say that a thing cannot be, which cannot be suitably. And so the comparison refers to what ought to be done, not to what is essential to be done; since water is of the essence of bread, but not of the essence of wine.
Reply to Objection 2. The shedding of the blood belonged directly to Christ's Passion: for it is natural for blood to flow from a wounded human body. But the flowing of the water was not necessary for the Passion; but merely to show its effect, which is to wash away sins, and to refresh us from the heat of concupiscence. And therefore the water is not offered apart from the wine in this sacrament, as the wine is offered apart from the bread; but the water is offered mixed with the wine to show that the wine belongs of itself to this sacrament, as of its very essence; but the water as something added to the wine.
Reply to Objection 3. Since the mixing of water with the wine is not necessary for the sacrament, it does not matter, as to the essence of the sacrament, what kind of water is added to the wine, whether natural water, or artificial, as rose-water, although, as to the propriety of the sacrament, he would sin who mixes any other than natural and true water, because true water flowed from the side of Christ hanging on the cross, and not phlegm, as some have said, in order to show that Christ's body was truly composed of the four elements; as by the flowing blood, it was shown to be composed of the four humors, as Pope Innocent III says in a certain Decree. But because the mixing of water with flour is essential to this sacrament, as making the composition of bread, if rose-water, or any other liquor besides true water, be mixed with the flour, the sacrament would not be valid, because it would not be true bread.
Source: Summa Theologica (New Advent)