Born Again: Water and Spirit

The Question:
What does the 'water' in John 3:5 refer to? Is it human birth?

Great question!
Firstly we can see that John has repeated this phrase from v3

I tell you the truth that no one can see the kingdom of God unless he is
born again

And v5

I tell you the truth, no one can enter the kingdom of God unless he is born of
water and the Spirit.

There are a raft of interpretations put forward by various people but only two seem main contenders:
1. The water can refer to human birth (as in the question) then it becomes a reference to the amniotic fluid that happens at childbirth. So, consequently there are two births, one natural and the other supernatural. However, this connection is not supported by any other places in Scripture to make it a viable option. Due to the close connection between v3 & 5 the water and Spirit need to be taken as a reference to the 'born again' (or as the footnote in the NIV says 'from above')
2. Water may refer to Christian baptism. A lot of commentaries go down this line and understandably because of the various references to baptism that surround this passage. However if the water = baptism and therefore a necessary role in entering the kingdom of God it is surprising that it is never mentioned again. But that is not to completely right this off altogether, there may be some allusion to baptism.

Having seen that neither of these two options are completely satisfying the third option seems to be the go
3. We have to take seriously the parallel with v3 'from above'. There seems to be a unity in how the words are put together that show that water and spirit are talking about the origin of regeneration (that is being born again). And Nicodemus is in trouble as Israel's teacher for not getting it, so the answer must lie somewhere in the Old Testament.
The Spirit in the Old Testament is God's principle of life and there will be a time when it is poured out upon all. It is a source of righteousness. Water is used figuratively to refer to renewal or cleansing. The two ideas come together in Ezekiel 36:25-27

I will sprinkle clean water on you, and you will be clean; I will cleanse
you from all your impurities and from all your idols. I will give you a new
heart and put a new spirit in you; I will remove from you your heart of stone
and give you a heart of flesh. And I will pour my Spirit in you and move you to
follow my decrees and be careful to keep my laws

This seems to depict individual and corporate regeneration (born again) was an idea presented strongly in the Old Testament. Regeneration that was of water and of the spirit. That is cleansed pure by the metaphoric washing of water and by the very real presence of the Holy Spirit (although not until Jesus returns to the Father read John 14-16 to see this).