Jesus answered her, "If you knew the gift of God and who it is that asks you for a drink, you would have asked him and he would have given you living water." John 4:10 NIV
First off here, Christ lifts this woman to a new, higher level of communication. He shows He is not limited by the prejudices of man.
Here came a woman to fetch water needed for natural life and is offered here a living water, that which allows the soul to be in communion with God. Such a much more needed water here than that for which the woman came. Christ has the power to draw this living water which the soul yearns to have, just as the woman had the pail to draw the water from the well to cure natural thirst.
So, what is the living water that Christ refers to? It is the Holy Spirit, who is given to those who believe in Christ. This is why Christ could say to the woman that He would have given to her living water if she knew who He was and had asked for it.
May the living water given by Christ, flow from the depths of your heart!