Doesn't that make baptism a work? Sure enough!

But some are accustomed to ask, "If baptism is itself a work and you say that works are of no use for salvation, what place is there for faith?" Answer: Yes, it is true that our works are of no use for salvation. Baptism, however, is not our work, but God's work (for, as was said, you must distinguish Christ's baptism quite clearly from a bath-keeper's baptism). God's works are salutary and necessary for salvation, and they do not exclude but rather demand faith, for without faith one cannot grasp them. Just by allowing the water to be poured over you, you do not receive or retain baptism in such a manner that it does you any good. But it becomes beneficial to you if you accept it as God's command and ordinance, so that, baptized in God's name, you may receive in the water the promised salvation.

-- Martin Luther, "Fourth Part: Concerning Baptism", Large Catechism