A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
isa 12:2THANKSGIVING HYMN OF THE RESTORED AND CONVERTED JEWS. (Isa 12:1-6)
Lord JEHOVAH--Jah, Jehovah. The repetition of the name denotes emphasis, and the unchangeableness of God's character.
strength . . . song . . . salvation--derived from Exo 15:2; Psa 118:14. The idea of salvation was peculiarly associated with the feast of tabernacles (see Isa 12:3). Hence the cry "Hosanna," "Save, we beseech thee," that accompanied Jesus' triumphal entry into Jerusalem on that day (the fifteenth of the seventh month) (Mat 21:9; compare with Psa 118:25-26); the earnest of the perfected "salvation" which He shall bring to His people at His glorious second appearance at Jerusalem (Heb 9:28). "He shall appear the second time without sin unto salvation." Compare Rev 21:3, "The tabernacle of God is with men." Compare Luk 9:33, "three tabernacles: one for thee," &c. (the transfiguration being a pledge of the future kingdom), (Psa 118:15; Zac 14:16). As the Jew was reminded by the feast of tabernacles of his wanderings in tents in the wilderness, so the Jew-Gentile Church to come shall call to mind, with thanksgiving, the various past ways whereby God has at last brought them to the heavenly "city of habitation" (Psa 107:7).
isa 12:3draw water . . . salvation--an expressive image in a hot country. On the last day of the feast of tabernacles the Jews used to bring water in a golden pitcher from the fountain of Siloam, and pour it, mingled with wine, on the sacrifice on the altar, with great rejoicing. This is the allusion in Jesus' words on "the last day of the feast" (Joh 7:2, Joh 7:37-39). The pouring out of water indicated repentance (Sa1 7:6; compare, as to the Jews' repentance hereafter, Zac 12:10). There shall be a latter outpouring of the Spirit like the former one on pentecost (Joe 2:23).
wells--not mere streams, which may run dry, but ever-flowing fountains (Joh 4:14; Joh 7:38), "Out of his belly (that is, in and from himself)--living water" (Isa 42:18; Psa 84:6; Zac 13:1; Rev 7:17).
isa 12:4make mention--Hebrew, "cause it to be remembered."
isa 12:5Sing, &c.--alluding to Exo 15:21.
isa 12:6inhabitant of Zion--Hebrew, "inhabitress"; so "daughter of Zion," that is, Zion and its people.
in the midst of thee--of Jerusalem literally (Jer 3:17; Eze 48:35; Zep 3:15, Zep 3:17; Zac 2:10).
The predictions as to foreign nations are for the sake of the covenant people, to preserve them from despair, or reliance on human confederacies, and to strengthen their faith in God: also in order to extirpate narrow-minded nationality: God is Jehovah to Israel, not for Israel's sake alone, but that He may be thereby Elohim to the nations. These prophecies are in their right chronological place, in the beginning of Hezekiah's reign; then the nations of Western Asia, on the Tigris and Euphrates, first assumed a most menacing aspect.