Certainly not just water, but the word of God in and with the water does these things, along with the faith which trusts this word of God in the water. For without God's word the water is plain water and no Baptism. But with the word of God it is a Baptism, that is, a life-giving water, rich in grace, and a washing of the new birth in the Holy Spirit, as St. Paul says in Titus, chapter three:
"He saved us through the washing of rebirth and renewal by the Holy Spirit, whom He poured out on us generously through Jesus Christ our Savior, so that, having been justified by His grace, we might become heirs having the hope of eternal life. This is a trustworthy saying."
--Small Catechism: Sacrament of Holy Baptism