31 And I knew him not: but that he should be made manifest to Israel, therefore am I come baptizing with water.
The sense is:
I did not know who he was to be, except simply that he was to be to be1 made manifest to Israel: therefore, in preparation for his coming,2 I have come to baptize with a baptism of repentance.3
2 Lk 3:11; Acts 19:4
John 1:33 states that the one who told him the Messiah would come baptizing with the Holy Ghost first told him to baptize with water, specifically:
John 1:33 (ESV) I myself did not know him, but he who sent me to baptize with water said to me, ‘He on whom you see the Spirit descend and remain, this is he who baptizes with the Holy Spirit.’
Thus, whoever sent John the Baptist set the precedent of baptism of water, not John.
God himself sent John the Baptist. Carefully read the following and note God have John his instructions.
"Now in the fifteenth year of the reign of Tiberius Caesar, Pontius Pilate being governor of Judea, Herod being tetrarch of Galilee, his brother Philip tetrarch of Iturea and the region of Trachonitis, and Lysanias tetrarch of Abilene, while Annas and Caiaphas were high priests, the word of God came to John the son of Zacharias in the wilderness. And he went into all the region around the Jordan, preaching a baptism of repentance for the remission of sins," Luke 3:1-3