Water has often been significant in God's dealings with His people.
He has used it for salvation and judgment: Righteous Noah and his family were saved from the judgment on a wicked world; Moses and his people were saved by water from a vengeful Pharoah and his army.
When John the Baptist came out of the wilderness, preaching and offering baptism, this was not a new thing. The idea of ritual washing for purification is as old as the laws of Moses. The tabernacle in the desert had a basin so that the priests could wash before entering. Solomon's temple had a large "sea" of water to be used in worship. Many of the rules and observances in the Law require ritual washing. The "mikvah", or pool, has been part of Jewish communities since before New Testament times; the ruins of Masada and Qumran, which date back to this area, have pools with provision for running water. Still today, Orthodox Jewish tradition calls for the construction of the mikvah even before a synagogue or temple.
In most cases, the ritual washings are repeated as the occasions recur. One exception is the baptism of a convert to Judaism, on their becoming a member of the family of Israel. This is similar to the Christian tradition--we baptize once, on the willing entry into God's family. The difference is similar to the tradition of atonement. The Old Testament describes the yearly celebration of Yom Kippur, the Day of Atonement, a yearly day of repentance. The New Testament doctrine of atonement is that the once for all death of Jesus Christ is adequate to atone for all sin, and our free choice of repentance and acceptance is a once for all event, followed by baptism, also a once for all acknowledgement of the death and resurrection of Jesus. We born a new creature, through the water of baptism and the blood of Jesus.