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;

Revelation 11:6

They have power to shut up the heaven, that the rain may not fall during the days of their prophecy: and they have power over the waters to turn them into blood, and to smite the earth with every plague, so often as they may wish.

Revelation 16:3-6

And the second poured out his bowl upon the sea; and it became as it were the blood of a dead man: and every soul of life died, whatsoever were in the sea.

Treasury of Scripture Knowledge

Summary

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.

Bible References

And they

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;
Revelation 16:3
And the second poured out his bowl upon the sea; and it became as it were the blood of a dead man: and every soul of life died, whatsoever were in the sea.