Exodus 7:14-25 -- The Water Turns to Blood

Exodus 7:14 Then the Lord said to Moses, “Pharaoh's heart is hardened; he refuses to let the people go. 15 Go to Pharaoh in the morning, as he is going out to the water. Stand on the bank of the Nile to meet him, and take in your hand the staff that turned into a serpent. 16 And you shall say to him, ‘The Lord, the God of the Hebrews, sent me to you, saying, “Let my people go, that they may serve me in the wilderness.” But so far, you have not obeyed. 17 Thus says the Lord, “By this you shall know that I am the Lord: behold, with the staff that is in my hand I will strike the water that is in the Nile, and it shall turn into blood. 18 The fish in the Nile shall die, and the Nile will stink, and the Egyptians will grow weary of drinking water from the Nile.”’” 19 And the Lord said to Moses, “Say to Aaron, ‘Take your staff and stretch out your hand over the waters of Egypt, over their rivers, their canals, and their ponds, and all their pools of water, so that they may become blood, and there shall be blood throughout all the land of Egypt, even in vessels of wood and in vessels of stone.’”
20 Moses and Aaron did as the Lord commanded. In the sight of Pharaoh and in the sight of his servants he lifted up the staff and struck the water in the Nile, and all the water in the Nile turned into blood. 21 And the fish in the Nile died, and the Nile stank, so that the Egyptians could not drink water from the Nile. There was blood throughout all the land of Egypt. 22 But the magicians of Egypt did the same by their secret arts. So Pharaoh's heart remained hardened, and he would not listen to them, as the Lord had said. 23 Pharaoh turned and went into his house, and he did not take even this to heart. 24 And all the Egyptians dug along the Nile for water to drink, for they could not drink the water of the Nile.
25 Seven full days passed after the Lord had struck the Nile.

Again Moses and Aaron showed Pharaoh a sign from God and again his magicians were able to duplicate it using "secret arts."  This was the first plague brought on Egypt by God to show his power and strength in judgment against a people who mistreated the Israelites.  All of the water turned to blood, the fish died and the river stunk.  They could not drink the water.  This is a big deal since the Nile was the main source of water for everyone in the area.  Since the rest of the Egyptian area was dessert, the Nile was necessary for life.

The turning the Nile into blood is similar to what will happen one day as Christ returns:

Revelation 16:4 The third poured out his bowl into the rivers and the springs of water, and they became blood. 5 I heard the angel of the waters say:
You are righteous,
who is and who was, the Holy One,
for You have decided these things.
6 Because they poured out
the blood of the saints and the prophets,
You also gave them blood to drink;
they deserve it!
7 Then I heard someone from the altar say:
Yes, Lord God, the Almighty,
true and righteous are Your judgments.

This is one of the judgments poured out on a sinful and wretched world.  Sinners have spilled the blood of the righteous, so God has given them blood to drink as a punishment.

Revelation 11:3 I will empower my two witnesses, and they will prophesy for 1,260 days, dressed in sackcloth.” 4 These are the two olive trees and the two lampstands that stand before the Lord of the earth. 5 If anyone wants to harm them, fire comes from their mouths and consumes their enemies; if anyone wants to harm them, he must be killed in this way. 6 These men have the power to close up the sky so that it does not rain during the days of their prophecy. They also have power over the waters to turn them into blood and to strike the earth with every plague whenever they want.   

At the end of times, the two witnesses will have the power to bring plagues upon the earth, much like the plagues that God brought on the Egyptians, one of which is turning the water into blood.