Albert Barnes Commentary Zechariah 14:8

"And it shall come to pass in that day, that living waters shall go out from Jerusalem; half of them toward the eastern sea, and half of them toward the western sea: in summer and in winter shall it be." — Zechariah 14:8 (ASV)

And it shall be, that living waters – Kimchi: “This is what is said in the prophecy of Joel, A fountain shall come forth from the house of the Lord (Joel 3:18); and in that of Ezekiel, And behold there ran out waters. (Ezekiel 47:2).” Zechariah leaves it to the mind to supply what the former prophets had said about the fertilizing, life-giving character of those waters.

He adds that they should pervade the whole land, west as well as east; to “the former,” rather “the Eastern Sea,” into which they would by nature flow, and toward “the hinder,” that is, the Western Sea, the Mediterranean, which natural waters could not “reach.” Their flow, he adds, should be perpetual: “These streams shall not dry up and their waters shall not fail”; therefore, drought shall not lessen them, nor winter-cold bind them. “From Jerusalem as from a fountain shall stream forth living waters of wisdom and grace to all nations.”

Cyril: “Again he tells us, under a figure, that exceedingly great and large will be that outpouring of the Holy Spirit upon the saints, especially when they will be removed to that holy eternal life in the world to come. For now, through faith in Christ we are enriched, as with an earnest, with the firstfruits of the Holy Spirit.

“But after the Resurrection, sin being wholly taken away, the Holy Spirit will be in us, not as an earnest or in a measure; but richly, bounteously, and perfectly will we enjoy the grace through Christ. He calls, then, living water, the Spirit which, he says, will come forth from the Jerusalem which is from above.

“But that the Holy Scripture is accustomed to liken the Divine Spirit to water, the Giver of it, the Son, confirms, saying, He that believeth on Me, as the Scripture hath said, Out of his belly shall flow rivers of living water (John 7:38). This the Evangelist explains: This spake He of the Spirit, which they who believe in Him should receive (John 7:39). Since then the Spirit is life-giving, rightly does He liken it to that which is life-giving to the body.”