The Spirit of God was moving over the face of the waters.—Genesis 1:2
The voice of the Lord is upon the waters: the God of glory thunders, the Lord, upon many waters. —Psalms 29:3
Purge me with hyssop, and I shall be clean; wash me, and I shall be whiter than snow. —Psalms 51:7
Holy Water is defined by Fr. John Hardon as "a sacramental blessed by a priest, invoking God's blessing on all who use it. Blessed water is a symbol of spiritual cleansing, and its use is advised in moments of physical danger and against temptations from spiritual enemies. It is common practice to dip one's fingers in Holy Water and reverently make the Sign of The Cross as one enters a Catholic church, and it is recommended for use in the home.
Blessing with Holy Water in the name of the Father, and of the Son and of the Holy Spirit is a powerful yet simple action. It reminds us of the cleansing given our souls through Baptism and it declares the truth of the Trinity. There are many ways to use Holy Water. It can be sprinkled in the home, on possessions, on vehicles or on the workspace.
St. Teresa of Avila used Holy Water often as she struggled with the devil. She noted in her autobiography "from long experience I have learned that there is nothing like Holy Water to put devils to flight and prevent them from coming back again."
Sacramentals do not confer the grace of the Holy Spirit in the way that the sacraments do, but by the Church's prayer, they prepare us to receive grace and dispose us to cooperate with it —(CCC 1670)