Jesus answered: Amen, amen I say to thee, unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God.
The correct quote is somewhat different. John 3:5, [Jesus answered, "I tell you the truth, unless a man is born of water and the spirit , he cannot enter the Kingdom of God.]
In verse 3 he told Nicodemus that he must be born again, but verse 5 says nothing about...again. All men are born of water....at birth, when the mother's water breaks. This is not referencing baptism. Not all men will be born again by the spirit....but when it happens, the ones born again will be invisible. (verse 8) Until that point (the resurrection) the proper term is "begotten" of the spirit....just like Jesus in verse 16....[For God so loved the world that He gave his only begotten Son .....] Even Jesus was not "Born Again" until his resurrection.
Jesus also said that the thief would be with Him in Paradise that day, without the benefit of water baptism. Baptism doesn't save you- rather, it's a prophetic, public declaration of what has already happened in the Spirit. We are baptized by the Spirit into Christ at the moment of salvation, this is the 'One Baptism' of salvation (one faith, one Lord, one baptism). Likewise, Jesus baptizes us with the Holy Spirit and fire, as John the Baptizer prophecied. This is believed by many to be a separate event apart from salvation, even as water baptism is.
Hebrews 6:2 speaks of baptisms, plural. Paul (probably the author) referenced it as a foundational doctrine upon which the faith is built.