Living waters shall go out - There shall be a wide diffusion of Divine knowledge, and of the plan of human salvation, which shall go out by apostles and preachers, first from Jerusalem, then to Syria, Asia Minor, Greece, Italy, the isles of the sea, Britain, etc.
The former sea, and - the hinder sea - The Dead Sea and the Mediterranean; see on Joel 2:20; (note). These are metaphors.
In summer - In time of drought; or in the countries where there was no knowledge of God, there shall these waters flow. The stream shall never cease; it shall run in summer as well as winter. These are living waters - perennial, incessant, and waters that shall preserve life. See John 7:37.
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 to the mind to supply what the former prophets had said of 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.” This 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 exceeding great and large shall be that outpouring of the Holy Spirit upon the saints, especially when they shall 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 shall we enjoy the grace through Christ. He calleth, 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 thereof, the Son, accredits, 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 frame.”