Summary: God turns the water into blood to show his power over Egypt and to begin the Exodus.
The last time we were together we read about when God turned Aaron’s staff into a serpent to prove his power and free Israel from Egyptian slavery. He had, however, already determined that Pharaoh wouldn’t listen, and that’s what happened when the magicians came into the room and performed the same wonder; Pharaoh’s hard heart got harder:
14And the LORD said unto Moses, Pharaoh’s heart is hardened, he refuseth to let the people go. 15Get thee unto Pharaoh in the morning; lo, he goeth out unto the water; and thou shalt stand by the river’s brink against he come; and the rod which was turned to a serpent shalt thou take in thine hand. 16And thou shalt say unto him, The LORD God of the Hebrews hath sent me unto thee, saying, Let my people go, that they may serve me in the wilderness: and, behold, hitherto thou wouldest not hear. 17Thus saith the LORD, In this thou shalt know that I am the LORD: behold, I will smite with the rod that is in mine hand upon the waters which are in the river, and they shall be turned to blood. 18And the fish that is in the river shall die, and the river shall stink; and the Egyptians shall lothe to drink of the water of the river.
Pharaoh’s refusal initiates the beginning of the ten plagues; Moses leaves the palace, and meets Pharaoh at the river’s bank the next morning with a warning: let the people go or this water turns to blood and everything in it dies.
I’ll keep reminding you that we want to be careful about giving symbolic meaning to the plagues. This is Moses’ first public miracle, and Jesus turned water into wine for his first public act, but the link isn’t stressed anywhere in the Old or New Testaments. Maybe they’re connected, but God doesn’t make a big dealing telling us about it. The main point that is expressed comes from verse seventeen: “In this thou shalt know that I am Yahweh.” This Yahweh puts a difference between Egypt and Israel as he uses one to display his power to destroy and the other his power to deliver.
Announcing the wonder before it happens shows that it’s not some freak accident, and it answers Pharaoh's question from 5:2. “Who is the LORD that I should obey his voice?” Well, this is the LORD, and Pharaoh is impotent to stop him. In fact, none of the Egyptian gods can do anything to prevent Yahweh from taking Egypt’s lifesource, and that’s the point: Yahweh is powerful to destroy.
He is also powerful to deliver, and that echoes in Psalm 78 where the writer emphasizes Israel’s cycles of salvation, unfaithfulness, and judgment. He walks through their history including the Exodus and all its wonders (v. 44), and observes that the Lord saved repeatedly “That they might set their hope in God” (v. 7).
This wonder is also mentioned in Psalm 105:29 to highlight God’s faithfulness despite Israel’s cycle of failures: “For he remembered his holy promise, and Abraham his servant. 43And he brought forth his people with joy, and his chosen with gladness: 44And gave them the lands of the heathen: and they inherited the labour of the people; 45That they might observe his statutes, and keep his laws. Praise ye the LORD” (Ps. 105:42-45).