Some say that the birth of water refers to the natural childbirth, but this is impossible. First of all, Jesus never explains a symbolic phrase with another symbolic phrase. The term “born again” is symbolic: Jesus was using the symbolism of a natural experience (birth) to help us understand a spiritual experience (salvation). This means that the phrase “born of water” cannot be symbolic and therefore must be taken as literal. We must literally be born (brought forth) of water. In addition, Jesus never mixes together that which is symbolic and that which is literal. If “born of the Spirit” is to be understood as literal, then “born of water” must be considered literal as well.
This brings us to the question: What does the 'birth of water' actually mean? We would say it refers to water baptism in the name of Jesus Christ. Compare John 3:5 with Acts 1:4-5, 2:38, 10:44-48, and 19:1-6.