Water Against Evil

God used water to destroy evil from the earth.

Genesis 6:17 And, behold, I, even I, do bring a flood of waters upon the earth, to destroy all flesh, wherein is the breath of life, from under heaven; and every thing that is in the earth shall die.

So it rained upon the earth forty days and forty nights; and every living substance was destroyed from the face of the earth.

Genesis 7:23 And every living substance was destroyed which was upon the face of the ground, both man, and cattle, and the creeping things, and the fowl of the heaven; and they were destroyed from the earth: and Noah only remained alive, and they that were with him in the ark.

John the Baptist also used water in Baptism to wash away sins.

The Acts 22:16 And now why tarriest thou? arise, and be baptized, and wash away thy sins, calling on the name of the Lord.

But what bothers me is the unclean spirits that enter & dwell in a man.

Matthew 12:43 When the unclean spirit is gone out of a man, he walketh through dry places, seeking rest,

If God saw that every thing (heaven & earth) he had made is good, where did the evil came from?

Genesis 1:31 And God saw every thing that he had made, and, behold, it was very good.

The evil came from Satan, who said in his heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north:

Isaiah 14:14 I will ascend above the heights of the clouds; I will be like the most High.

Evil therefore came from Satan not on the earth where good man dwells.

Job 1:7 And the LORD said unto Satan, Whence comest thou? Then Satan answered the LORD, and said, From going to and fro in the earth, and from walking up and down in it.

The reason why we are advised to be sober, watchful, and firm in our faith.

1 Peter 5:8 Be sober, be vigilant; because your adversary the devil, as a roaring lion, walketh about, seeking whom he may devour:

When unclean spirits walketh through dry places, why it enter & dwell in a man where man's body is scientifically proven as composed mostly of water.

The water in this instant case is the word of GOD where unclean spirits can never dwell.

John 7:38 He that believeth on me, as the Scripture hath said, out of his belly shall flow rivers of living water.

Hebrews 4:12 For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.

James 4:7 Submit yourselves therefore to God. Resist the devil, and he will flee from you.

Ephesians 4:26 Be ye angry, and sin not: let not the sun go down upon your wrath:

Ephesians 4:27 Neither give place to the devil.

If he findeth it empty, swept, and garnished. Then goeth he, and taketh with himself seven other spirits more wicked than himself, and they enter in and dwell there: and the last state of that man is worse than the first.

Can a "holy water" cast out demons like what they are doing in exorcism?

Definitely not. You can spray all the water in the ocean and it will never cast out demons. What cast out demons is the word of God.
Matthew 8:31 So the devils besought him, saying, If thou cast us out, suffer us to go away into the herd of swine.

The presence of Jesus in this settings don't automatically cast them out until Jesus said unto them to go.

Matthew 8:32 And he said unto them, Go. And when they were come out, they went into the herd of swine: and, behold, the whole herd of swine ran violently down a steep place into the sea, and perished in the waters.

See, swine can't bear with them but man let them in.