Charles Ellicott Commentary Isaiah 12:3

"Therefore with joy shall ye draw water out of the wells of salvation." — Isaiah 12:3 (ASV)

Therefore with joy you shall draw water ... —Literally, And with joy. The words may be either part of the hymn, or addressed to those who are to join in it. The latter seems most in harmony with the context. In the later ritual of the Feast of Tabernacles, the priests went in solemn procession to the Pool of Siloam, filled a golden vase with water, carried it to the Temple, and poured it out on the western side of the altar of burnt offering, while the people chanted the great Hallel (Hymn of Praise) of Psalms 113-118 (See Note on John 7:37).

If we may assume that this represented the ritual of the monarchy, we may reasonably infer that the words of Isaiah pointed to it. The Talmud expressly connects the act with the symbolism of Isaiah’s words (Jerusalem Talmud, Sukkah 5:1), and the prophet’s reference to the waters of Shiloah in Isaiah 8:6 confirms the inference.