Phillip and he went down into the water and came up out of the water. Signifying that they went and stood in the water. When John the Baptist baptized in Aenon he did so because there was much water there . Much water was required because he dipped or immersed or buried them in the water. There are many scriptural reasons for baptism.
Baptism is for the remission of sin.
Jesus said that repentance and remission of sin should be preached in His name beginning at Jerusalem. There is a difference in forgiveness of sin and remission of sin. We are forgiven at repentance. At Jerusalem in Acts chapter 2, Peter said, Repent and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. Water Baptism is part of the New Birth experience.