14 ¶ And the LORD said unto Moses, Pharaoh’s heart is hardened, he refuseth to let the people go. 15 Get 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. 16 And 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. 17 Thus 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. 18 And 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. 19 And the LORD spake unto Moses, Say unto Aaron, Take thy rod, and stretch out thine hand upon the waters of Egypt, upon their streams, upon their rivers, and upon their ponds, and upon all their pools of water, that they may become blood; and that there may be blood throughout all the land of Egypt, both in vessels of wood, and in vessels of stone. 20 And Moses and Aaron did so, as the LORD commanded; and he lifted up the rod, and smote the waters that were in the river, in the sight of Pharaoh, and in the sight of his servants; and all the waters that were in the river were turned to blood. 21 And the fish that was in the river died; and the river stank, and the Egyptians could not drink of the water of the river; and there was blood throughout all the land of Egypt. 22 And the magicians of Egypt did so with their enchantments: and Pharaoh’s heart was hardened, neither did he hearken unto them; as the LORD had said. 23 And Pharaoh turned and went into his house, neither did he set his heart to this also. 24 And all the Egyptians digged round about the river for water to drink; for they could not drink of the water of the river. 25 And seven days were fulfilled, after that the LORD had smitten the river.
It was apparently the custom to go down the all-important Nile in the morning for bathing and perhaps ritual in the morning. There they met the Pharaoh. Between Moses and Aaron all of the surface water in Egypt was turned into blood. The only drinkable water was found by digging. Fish died and the Nile River stunk. This second wonder the heathen magicians could also duplicate which did not support Moses’ and Aaron’s credibility. This went on for seven days.
Psalm 78:44 And had turned their rivers into blood; and their floods, that they could not drink.
Psalm 105:29 He turned their waters into blood, and slew their fish.
This sort of judgment will come again at the end of human history, perhaps referring to the Mediterranean Sea.
Revelation 8:8 And the second angel sounded, and as it were a great mountain burning with fire was cast into the sea: and the third part of the sea became blood; 9 And the third part of the creatures which were in the sea, and had life, died; and the third part of the ships were destroyed.
The Pharaoh’s heart is hardened and he will not listen to this warning. Later, after all of their interactions with God and after so many of their distresses that arose from disobeying Him, it will be said;
Zechariah 7:12 Yea, they made their hearts as an adamant stone, lest they should hear the law, and the words which the LORD of hosts hath sent in his spirit by the former prophets: therefore came a great wrath from the LORD of hosts.