In the liturgy the priest prays " Likewise also the cup after supper He mixed it of wine and water. He gave thanks.."
If I am not mistaken,the bible does not say that Jesus Himself mixed wine with water.Therefore,if that is so, we do not know for sure,unless we rely on the customs and traditions of wine mixing of the day.Perhaps,the practice of adding water has symbolic significance , but I do not think that it is necessary for the validity of the Holy Sacrament. I do not know if the mixing of water is in reference to the verse in (Proverbs 9:5): 'Drink the wine which I have mixed for you.? However,why is it important to mix the wine with water, bearing in mind,that there is already large amount of water in the finished product of wine?