3:1 "Now there was a man of the Pharisees, named Nicodemus, a ruler of the Jews...."
Because of his background, he would have been well versed in OT scripture. It would be easy to see how Nicodemus understood the meaning of water in this statement from Jesus.
Nicodemus said to Him, "How can a man be born when he is old? He cannot enter a second time into his mother's womb and be born, can he?" Jesus answered, "Truly, truly, I say to you, unless one is born of water and the Spirit he cannot enter into the kingdom of God.
Ezek 36:24-28 "For I will take you from the nations, gather you from all the lands and bring you into your own land. "Then I will sprinkle clean water on you, and you will be clean; I will cleanse you from all your filthiness and from all your idols. "Moreover, I will give you a new heart and put a new spirit within you; and I will remove the heart of stone from your flesh and give you a heart of flesh. "I will put My Spirit within you and cause you to walk in My statutes, and you will be careful to observe My ordinances. "You will live in the land that I gave to your forefathers; so you will be My people, and I will be your God.
Jer 31:31,33 Behold, days are coming," declares the LORD, "when I will make a new covenant with the house of Israel and with the house of Judah, "But this is the covenant which I will make with the house of Israel after those days," declares the LORD, " I will put My law within them and on their heart I will write it; and I will be their God, and they shall be My people.
Isaiah 44:3-5 and Ezek 37:9-10 are pertinent examples of water and wind as life-giving symbols of the Spirit of God in his work among people. Both occur in contexts that deal with the future restoration of Israel as a nation prior to the establishment of the messianic kingdom. It is therefore particularly appropriate that Jesus should introduce them in a conversation about entering the kingdom of God. Both water and wind are figures (based on passages in the OT, which Nicodemus, the teacher of Israel should have known) that represent the regenerating work of the Spirit in the lives of men and women.