Isaiah 55:1

"The introductory particle (hoi) is mainly an attention-getting device, but it expresses a slight tone of pity. The prophet is an evangelist with a concern for the souls of men and a realization of their desperate condition without the blessings that the servant has obtained." [Note: Ibid.]

After getting their attention, Isaiah, speaking as God and for God, called the thirsty to come and drink freely, and to the hungry to enjoy a free meal (cf. Proverbs 9:5-6; Matthew 5:6; John 4:13-14; John 6:32-35; Revelation 22:17). Water, that formerly represented the Holy Spirit (cf. Isaiah 32:15; Isaiah 44:3), was now available to the people because of the Servant’s work. Jesus extended a similar invitation to those in His day to come to Him to receive this water (i.e., eternal life through the Spirit; John 4:10-14; John 7:37-38). The Lord’s offer was to buy what was free. The only way to do this is to use someone else’s money to purchase it. It was the Servant’s payment for sin that made salvation free for those who count His "money" good.

"The abundance and freeness of the water of refreshment (Isaiah 44:3), the wine of joy (Isaiah 25:6-8) and the milk of richness ([nourishment] Exodus 3:8) and supremacy (Isaiah 60:16) is figurative of the Lord’s salvation with the Servant at its centre (see Isaiah 55:3-5)." [Note: Motyer, p. 453.]