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.
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.
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.
And the magicians of Egypt did so with their enchantments....
Exodus 7:20-22
"The 1st plague was directed against the Nile itself, which
the 
Egyptians worshiped as their source of life.
This plague confronted
the numerous river deities, including Khnum (guardian of the Nile),
Hapi (spirit of the Nile and god of fertility) and Osiris (god of the
underworld, whose blood the Nile was considered to be).
By turning the
Nile into blood and killing the fish (supposedly protected by Hathor and
Neith), Moses was not only shaming all these Egyptian objects of
worship, but also showing that sustenance comes only from the hand of
Yahweh." CMI