Water baptism is a Jewish tradition that the Jews still practice today. It started in Leviticus where all the different ways to become spiritually unclean is given. In the middle of all these ways to become unclean is a scripture that explains spiritual cleaning with running water. Today, many Jewish synagogues have pools of water called mikvas where baptism occurs for spiritual cleansing. That is why Jews from all over went to John's baptism. They wanted to become spiritually clean.
John said that he baptized with water, but there will be someone who will be baptizing with the Holy Spirit of God instead of water.
The first Christians were devout Jews who still celebrated Jewish traditions such as the Passover, circumcision, baptism , etc. When they figured out that Gentiles were included in salvation, they at first wanted the Gentiles to follow their same traditions and practices such as circumcision and baptism. They eliminated circumcision for the Gentiles, but the water baptism remained because the tradition was so strong and prevalent. John said that it will be Christ who will be baptizing with the Holy Spirit and not water.
So what we have now is man following John's baptism with water and declaring that that is when the person is born again and that is what all Christians must do. Yes baptism is a Greek word that means water immersion, but Jesus used the word differently in Matt 20: 22 when He told His apostles that He had a baptism that He had to go through. It is God who does the actual baptizing, not man. In the great commission, Jesus tells His Apostles to " Go ye, therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit, ". The baptism occurs automatically by Christ through the Apostle's teaching. He didn't tell them to carry a bucket of water in case no water is available.
Matt 12: 41 The men of Nineveh shall rise in judgment with this generation, and shall condemn it; because they repented at the preaching of Jonah; and, behold, a greater than Jonah is here.
1 Cor 1:21 For after that, in the wisdom of God, the world by wisdom knew not God, it pleased God by the foolishness of preaching to save them that believe.
1 Cor 1: 17 For Christ sent me not to baptize but to preach the gospel; not with wisdom of words, lest the cross of Christ should be made of no effect.
1 Cor 1: 18 For the preaching of the cross is to them that perish foolishness; but unto us who are saved it is the power of God.