The story of Gideon and the dew upon the fleece from Judges 6:36-40 is strongly associated with the Incarnation of Christ in Orthodox Christian hymnody. From the First Canon of Christmas (Ode Four):
Like rain upon a fleece, You came down into the Virginal womb, O Christ, and like drops that fall upon the earth.
From the Canon of the Annunciation (Ode Four):
The Word of the Father shall descend upon you like rain upon a fleece, even as it has seemed good to Him.
I think, however, that Ambrose is also using this image of water to suggest an association with baptism:
There is also a certain water which we put into the basin of our soul, water from the fleece and from the Book of Judges; water, too, from the Book of Psalms.* It is the water of the message from heaven.
*[The Lord] leads me beside still waters — Psalm 22 (23):2
Of course, we strongly associate baptism with the descent of the Holy Spirit (from the Blessing of the Baptismal Water):
Will You also now, Loving King, be present through the descent of Your Holy Spirit and sanctify this water.
Thus, St. Ambrose connects the Incarnation with both the Holy Spirit (who descended upon the Virgin Mary) and baptism (when the Holy Spirit descends upon the water). In this light, it is entirely appropriate to pray:
Let, then, this water, O Lord Jesus, come into my soul, into my flesh, that through the moisture of this rain the valleys of our minds and the fields of our hearts may grow green. May the drops from Thee come upon me, shedding forth grace and immortality. Wash the steps of my mind that I may not sin again. Wash the heel, of my soul, that I may be able to efface the curse, that I feel not the serpent’s bite on the foot of my soul, but, as Thou Thyself hast bidden those who follow Thee, may tread on serpents and scorpions with uninjured foot. Thou hast redeemed the world, redeem the soul of a single sinner.
In other words, it is through the descent of the Holy Spirit that our soul and our very flesh has access to the salvation granted by the Incarnation of Christ — in whom the Protoevangelium (first good news) of Genesis 3:15 becomes flesh:
I will put enmity between you [the serpent] and the woman [Eve], and between your seed and her seed. He shall bruise your head, and you shall be on guard for His heel.
Amen. Amen. Amen.