From Pastor Paul Holt:
This is still the week of Sukkot, it ends Thursday. That was when Jesus said this: In the last day, that great day of the feast, Jesus stood and cried, saying, “If any man thirst, let him come unto me, and drink. He that believeth on me 
as the scripture hath said, out of his belly shall flow rivers of living water.” John 7: 37-38, KJV
The ceremonial water of the Feast symbolized the Holy Spirit. A link between water and the Holy Spirit is implied in the parallel structure of Isaiah 44:3: “For I will pour water upon him that is thirsty, and floods upon the dry ground: I will pour my spirit upon thy seed, and my blessing upon thine offspring.” When the people prayed for rain at the Feast, they may also have prayed for the rain of the Holy Spirit to fall as was prophesied in Joel 2:28-29.
