Is baptism by water, a Jewish tradition?

After a quick search, I have found it is  jewish tradition. 

No.  You mean:  Is baptism by what [no comma] a Jewish tradition?

God sent John the Baptist to prepare the way of the Lord Jesus Christ who was to come.  All those in Israel came to John's baptism and repented of their sins in their baptism in water.  Then their hearts were ready to receive the Saviour of the World, Jesus Christ when He came.

Prior to John the Baptist, there were no baptisms in the Old Testament.  That baptism of John was specifically for repentance so they could receive their Messiah when He came.  Those baptisms were into John - they became disciples of John the Baptist.

Once Jesus Christ came and preached the gospel He told us to preach the same Gospel He did, and baptize those who believe  -  not into John the Baptist anymore, but in the Name of the Father, Son and Holy Spirit. (Matthew 28: 19 & 20) Those who repent and get baptized in the name of the Father, Son and Holy Spirit have remission of sins and shall receive the gift of the Holy Spirit (Acts 2:38)

Basically that means that you do not choose to be baptized by the Holy Spirit. He chooses you from birth because John the baptist was told by the Holy Spirit to do that. He was baptized by the Holy Spirit when his mother was when he was inside his mother. So the choice was not his.

John the baptiser who baptised Jesus with water  prior to Jesus receiving Holy Spirit baptised Jews who had repented refusing to baptise scibes and pharisees according to the bible Math chapt 3

The main difference is that in Orthodox Judaism, a ritual cleansing is something to be repeated frequently, but especially for women after their periods and before having sex with their husbands.  Men may do it before the beginning of the Sabbath, so that would be weekly, but some will do it every day.

Answered as to the purpose, Luke 3:3, Mark 1:4.

John the Baptist was Jewish.  He was preparing for a Messiah.