I will pour water upon him that is thirsty. "Water" is, in Isaiah, the common metaphor for Divine grace. Sometimes, as in this place (and Isaiah 35:6; Isaiah 43:20; Isaiah 55:1), the simple maim, "water" or "waters," is the word used.
At other times we have instead, or in addition, "rain" (Isaiah 5:6; Isaiah 30:23; Isaiah 55:10), or "dew" (Isaiah 26:19), or "rivers" (Isaiah 30:25; Isaiah 32:2; Isaiah 33:21; Isaiah 41:18; Isaiah 43:19, etc.
), or "streams" (Isaiah 30:25; Isaiah 35:6), or "floods" (as in this place). At his coming on earth, our blessed Lord took up the comparison, and has made it familiar to all men throughout the whole Christian world (see John 3:5; John 4:10, John 4:11, John 4:13-15; John 7:37-39).
We may note here that the "water" is only poured on him who is athirst for it. Thy seed … thine offspring. Not "Israel after the flesh" only, but also Israel after the Spirit—the true "Israel of God" (Galatians 6:16).