The most ancient testimony of the ancient prayer, in the Byzantine Rite, of the blessing of the baptismal water, is found in Codex Vaticanus Barberino Graeco 336, p. 201, and it another early witness of baptismal regeneration. One version of this prayer is reproduced by a Byzantine Catholic Website, The Feast of the Theophany Part VII: Great Blessing of Water:
The Blessing of the Water
Three times, the priest makes the sign of the cross in the water with a three-branched candle (signifying the Holy Trinity), saying each time:
Great are you, O Lord, and wonderful are your works; no word suffices to give praise to your wonders.
Then he prays aloud, recounting the many reasons we have to praise God, in company with the angels and all creation. Breathing upon the water three times in the form of a cross, he says each time:
O Loving King, come now and through the descent of the Holy Spirit sanctify this water.
The breath is an ancient symbol of life bestowed; the Greek word for "spirit" is the same as "breath"; our Lord breathed on his disciples when he said, "Receive the Holy Spirit."
Then the priest prays aloud, asking God to bless the waters, and giving the reason for our hope and expectation of such a blessing:
Grant it the grace of redemption and the blessing of the Jordan.
Make it a fount of incorruptibility, a gift of sanctification, a redemption of sins,
a healing potion for illness, and a destroyer of demons.
Make it immune to hostile powers,
and fill it with angelic power so that all who drink and receive of it may be purified in soul and body,
cured of ills, sanctified in their homes, and given every befitting grace.
For you are our God, who through water and the Spirit rejuvenated our nature grown old by sin.
You are our God, who drowned sin in the waters at the time of Noah.
You are our God, who, on the sea and at the hands of Moses, delivered the Hebrews from the bondage of Pharaoh.
You are our God, who split the rock in the wilderness, so that the waters gushed out,
and the valleys overflowed, and the people were satisfied.
You are our God, who, with fire and water, and at the hands of Elijah, delivered Israel from the errors of Baal.
Make it a fount of incorruptibility, a gift of sanctification, a redemption of sins,
a healing potion for illness, and a destroyer of demons.
Make it immune to hostile powers,
and fill it with angelic power so that all who drink and receive of it may be purified in soul and body,
cured of ills, sanctified in their homes, and given every befitting grace.
For you are our God, who through water and the Spirit rejuvenated our nature grown old by sin.
You are our God, who drowned sin in the waters at the time of Noah.
You are our God, who, on the sea and at the hands of Moses, delivered the Hebrews from the bondage of Pharaoh.
You are our God, who split the rock in the wilderness, so that the waters gushed out,
and the valleys overflowed, and the people were satisfied.
You are our God, who, with fire and water, and at the hands of Elijah, delivered Israel from the errors of Baal.
Finally, the priest makes the sign of the cross in the water with his hand, three times, saying each time:
Wherefore, O Master, sanctify this water by your Holy Spirit.
and prays aloud:
Grant sanctification, blessing, cleansing, and health
to all who touch it, are blessed with it, or who partake of it.
O Lord, save your servants, our civil authorities.
Keep them in peace within your protective shadow, granting them all salutary requests and eternal life.
May your all-holy name be glorified by the elements, by men, by angels, by all that is visible or invisible,
together with the Father and the Holy Spirit, now and ever and forever.
to all who touch it, are blessed with it, or who partake of it.
O Lord, save your servants, our civil authorities.
Keep them in peace within your protective shadow, granting them all salutary requests and eternal life.
May your all-holy name be glorified by the elements, by men, by angels, by all that is visible or invisible,
together with the Father and the Holy Spirit, now and ever and forever.
and the people respond, "Amen." The blessing of the waters is complete.
For further evidence, both biblical and patristic, supporting baptismal regeneration, a topic one has disussed many times on this blog, see, for e.g.: