Psalm 105:29 He turned their waters into blood, and slew their fish.
Exodus 7:17-21 Thus said the LORD, In this you shall know that I am the LORD: behold...
Revelation 16:3-6 And the second angel poured out his vial on the sea; and it became as the blood of a dead man: and every living soul died in the sea...
The miracles mentioned in this and the four subsequent verses, evidently shew the power of God over the elements of nature, which at that time were the objects of Egyptian worship.