A.T. Robertson Commentary Mark 1:8

"I baptized you in water; But he shall baptize you in the Holy Spirit." — Mark 1:8 (ASV)

With water (υδατ). So Luke (Luke 3:16) the locative case,

in water . Matthew (Matthew 3:11) has εν (in), both with (in) water and the Holy Spirit. The water baptism by John was a symbol of the spiritual baptism by Jesus.