Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and [of] the Spirit, he cannot enter into the kingdom of God.
Note 4 at Joh 3:5: Being born of water is not speaking of water baptism as being essential for salvation. People were born again before they were baptized in water (Joh 20:28 with Ro 10:9). Cornelius and his household received the Holy Spirit before they were baptized in water (Ac 10:44-48), as evidenced by the gifts of the Holy Spirit (Ac 10:45) whom Jesus said the world (lost) could not receive (Joh 14:17). When Peter defended his visit to Cornelius before the church in Jerusalem (Ac 11:1-18), he was trying to prove to the Jewish believers that God had granted salvation to the Gentiles also. He used the fact that they had received the Holy Spirit as proof of their new birth, but he didn't mention water baptism. If the early church had used water baptism to produce salvation, Peter certainly would have been questioned about this.
In 1Co 1:17, Paul said, "Christ sent me not to baptize, but to preach the gospel." While in no way discrediting water baptism, Paul stated that it was not the Gospel. Some groups today spend much more time preaching water baptism than they do proclaiming the redemptive work of Jesus Christ.
This water could be speaking of the Word of God (Joh 15:3, Eph 5:26, Jas 1:18, and 1Pe 1:23) or the natural birth of a baby. Joh 3:6 might support the water as meaning the natural birth; however, either of these two interpretations are consistent with Scripture and do not pose any inconsistencies, as would water baptism.