Revelation 8:8

The second angel blew, and what looked like a huge mountain on fire was hurled into the sea; a third of the sea turned blood,

Revelation 11:6

They have power to shut up the sky, so that no rain falls during the days when they are prophesying; and they have power over the waters, to turn them into blood, and also to smite the earth with all manner of plagues as often as they choose.

Revelation 16:3-6

The second poured out his bowl upon the sea; it turned blood like the blood of a corpse, and every living thing within the sea perished.

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
The second angel blew, and what looked like a huge mountain on fire was hurled into the sea; a third of the sea turned blood,
Revelation 16:3
The second poured out his bowl upon the sea; it turned blood like the blood of a corpse, and every living thing within the sea perished.