Often parishes will have a blessing of waters outside also. Here the same parish has an outdoor blessing at a lake:
Fr. Thomas Hopko explains the theology behind this:
After the epistle (1 Cor 1:10-14) and the gospel reading (Mk 1:9-11) the special great litany is chanted invoking the grace of the Holy Spirit upon the water and upon those who will partake of it. It ends with the great prayer of the cosmic glorification of God in which Christ is called upon to sanctify the water, and all men and all creation, by the manifestation of his saving and sanctifying divine presence by the indwelling of the Holy and Good and Life-creating Spirit.
As the troparion of the feast is sung, the celebrant immerses the Cross into the water three times and then proceeds to sprinkle the water in the four directions of the world. He then blesses the people and their homes with the sanctified water which stands for the salvation of all men and all creation which Christ has effected by his “epiphany” in the flesh for the life of the world.
Sometimes people think that the blessing of water and the practice of drinking it and sprinkling it over everyone and everything is a “paganism” which has falsely entered the Christian Church. We know, however, that this ritual was practiced by the People of God in the Old Testament, and that in the Christian Church it has a very special and important significance.
It is the faith of Christians that since the Son of God has taken human flesh and has been immersed in the streams of the Jordan, all matter is sanctified and made pure in him, purged of its death-dealing qualities inherited from the devil and the wickedness of men. In the Lord’s epiphany all creation becomes good again, indeed “very good,” the way that God himself made it and proclaimed it to be in the beginning when “the Spirit of God was moving over the face of the waters” (Gen 1:2) and when the “Breath of Life” was breathing in man and in everything that God made (Gen 1:30; 2:7).
The world and everything in it is indeed “very good” (Gen 1:31) and when it becomes polluted, corrupted and dead, God saves it once more by effecting the “new creation” in Christ, his divine Son and our Lord by the grace of the Holy Spirit (Gal 6:15). This is what is celebrated on Epiphany, particularly in the Great Blessing of Water. The consecration of the waters on this feast places the entire world — through its “prime element” of watering the perspective of the cosmic creation, sanctification, and glorification of the Kingdom of God in Christ arid the Spirit. It tells us that man and the world were indeed created and saved in order to be “filled with all the fullness of God” (Eph 3:19), the “fullness of him who fills all in all” (Eph 1:22). It tells us that Christ, in who in “the whole fulness of deity dwells bodily,” is and shall be truly “all, and in all” (Col 2:9, 3:11). It tells us as well that the “new heavens and the new earth” which God has promised through his prophets and apostles (Is 66:2; 2 Peter 3:13, Rev 21:1) are truly “with us” already now in the, mystery of Christ and his Church.
Thus, the sanctification and sprinkling of the Epiphany water is no pagan ritual. It is the expression of the most central fact of the Christian vision of man, his life and his world. It is the liturgical testimony that the vocation and destiny of creation is to be “filled with all the fullness of God” (Eph 3:19).