Objection 1. It seems that water ought to be added in great quantity, because as blood flowed sensibly from Christ's side, so did water: hence it is written (John 19:35): "He that saw it, hath given testimony." But water could not be sensibly present in this sacrament except it were used in great quantity. Consequently it seems that water ought to be added in great quantity.
Objection 2. Further, a little water mixed with much wine is corrupted. But what is corrupted no longer exists. Therefore, it is the same thing to add a little water in this sacrament as to add none. But it is not lawful to add none. Therefore, neither is it lawful to add a little.
Objection 3. Further, if it sufficed to add a little, then as a consequence it would suffice to throw one drop of water into an entire cask. But this seems ridiculous. Therefore it does not suffice for a small quantity to be added.
On the contrary, It is said in the Decretals (Extra, De Celeb. Miss.): "The pernicious abuse has prevailed in your country of adding water in greater quantity than the wine, in the sacrifice, where according to the reasonable custom of the entire Church more wine than water ought to be employed."
I answer that, There is a threefold opinion regarding the water added to the wine, as Pope Innocent III says in a certain Decretal. For some say that the water remains by itself when the wine is changed into blood: but such an opinion cannot stand, because in the sacrament of the altar after the consecration there is nothing else save the body and the blood of Christ. Because, as Ambrose says in De Officiis (De Mysteriis ix): "Before the blessing it is another species that is named, after the blessing the Body is signified; otherwise it would not be adored with adoration of latria." And therefore others have said that as the wine is changed into blood, so the water is changed into the water which flowed from Christ's side. But this cannot be maintained reasonably, because according to this the water would be consecrated apart from the wine, as the wine is from the bread.
And therefore as he (Innocent III, Decretals, Extra, De Celeb. Miss.) says, the more probable opinion is that which holds that the water is changed into wine, and the wine into blood. Now, this could not be done unless so little water was used that it would be changed into wine. Consequently, it is always safer to add little water, especially if the wine be weak, because the sacrament could not be celebrated if there were such addition of water as to destroy the species of the wine. Hence Pope Julius I reprehends some who "keep throughout the year a linen cloth steeped in must, and at the time of sacrifice wash a part of it with water, and so make the offering."
Reply to Objection 1. For the signification of this sacrament it suffices for the water to be appreciable by sense when it is mixed with the wine: but it is not necessary for it to be sensible after the mingling.
Reply to Objection 2. If no water were added, the signification would be utterly excluded: but when the water is changed into wine, it is signified that the people is incorporated with Christ.
Reply to Objection 3. If water were added to a cask, it would not suffice for the signification of this sacrament, but the water must be added to the wine at the actual celebration of the sacrament.
Source: Summa Theologica (New Advent)