Revelation 8:8

And the second angel sounded; and the appearance of a great mountain burning with fire was cast into the sea; and a third part of the sea became blood,

Revelation 11:6

These have power to shut heaven, that it may not rain in 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 as often as they will.

Revelation 16:3-6

And the second poured out his bowl on the sea; and it became as the blood of a dead body, and every living soul died, the [creatures] 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 the appearance of a great mountain burning with fire was cast into the sea; and a third part of the sea became blood,
Revelation 16:3
And the second poured out his bowl on the sea; and it became as the blood of a dead body, and every living soul died, the [creatures] in the sea.