Continuing on the subject of water...
35: The Jews said to one another, "Where does this man intend to go that we shall not find him? Does he intend to go to the Dispersion among the Greeks and teach the Greeks?
36: What does he mean by saying, `You will seek me and you will not find me,' and, `Where I am you cannot come'?"
37: On the last day of the feast, the great day,
The feast is the Festival of Tabernacles, or Festival of Booths (Sukkot in Hebrew). The "great day" is the eighth day, Shemini Atzeret, which transcends the 7 day festival. On this day there is public prayer of the Geshem (גשם), an evocation for rain after the dry months.
37: On the last day of the feast, the great day, Jesus stood up and proclaimed, "If any one thirst, let him come to me and drink.
38: He who believes in me, as the scripture has said, `Out of his heart shall flow rivers of living water.'"
39: Now this he said about the Spirit, which those who believed in him were to receive; for as yet the Spirit had not been given, because Jesus was not yet glorified.