On the last and greatest day of the festival, Jesus stood and said in a loud voice, “Let anyone who is thirsty come to me and drink. Whoever believes in me, as Scripture has said, rivers of living water will flow from within them.” By this he meant the Spirit, whom those who believed in him were later to receive. Up to that time the Spirit had not been given, since Jesus had not yet been glorified.
John spells the metaphor out on plain speech. The Living Water is the Holy Sprit. On each of the days of the feast of Tabernacles, a golden flagon filled with water from the Pool of Siloam was carried by the High Priest back to the Temple where the water was offered to God at the time of the morning sacrifice.