A priest has received special blessings at his ordination (Sacrament of Holy Orders) as a representative of Christ to bless the water.
(I'm sure you already knew that -- and are just choosing to present the other side -- flase though it might be.
I am assuming that your church believes this supernatural power is a product of your theory of apostolic succession. If your theory of apostolic succession was shown to be incorrect would you then question all the other supernatural powers your priests are supposed to have?