Why does the sea turn to blood?
The sea turns to blood because God pours out the second vial (bowl) of His wrath directly upon it, an act of divine judgment that kills every living creature in the sea.
Scripture teaches that the sea turns to blood when the second angel pours out his vial upon it, as described in Revelation 16:3: "And the second angel poured out his vial upon the sea; and it became as the blood of a dead man: and every living soul died in the sea." This is not a natural event but a direct act of God's wrath being poured out on a rebellious world. The blood is described as thick, dark, and having a putrid odor, like the blood of a dead man, and the result is that every living creature in the sea dies.
This judgment is shown to be a progression of an earlier judgment. In the second trumpet judgment, only one third of the sea became blood and one third of the sea life died. Now, in this vial of wrath, the entire sea becomes as the blood of a dead man and all life in the seas perishes. It is the fuller, undiluted expression of the same kind of judgment seen earlier, showing the increasing severity of God's dealings with the earth.
This plague also has a precedent from the past. In the days of Moses, God turned the waters of Egypt to blood as a sign and a judgment against Pharaoh. Exodus 7:17-21 records that the Lord told Moses the waters of the river would be turned to blood, the fish would die, and the river would stink so badly that the Egyptians could not drink from it. Moses and Aaron carried this out exactly as the Lord commanded, and blood was found throughout all the land of Egypt. The sea and waters turning to blood in Revelation is presented as a similar, but far greater and final, act of God's judgment upon the whole earth.
The scriptures
And the second angel poured out his vial upon the sea; and it became as the blood of a dead man: and every living soul died in the sea.
And the third angel poured out his vial upon the rivers and fountains of waters; and they became blood. And I heard the angel of the waters say, Thou art righteous, O Lord, which art, and wast, and shalt be, because thou hast judged thus. For they have shed the blood of saints and prophets, and thou hast given them blood to drink; for they are worthy. And I heard another out of the altar say, Even so, Lord God Almighty, true and righteous are thy judgments.
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. And the fish that is in the river shall die, and the river shall stink; and the Egyptians shall lothe to drink of the water of the river. And the LORD spake unto Moses, Say unto Aaron, 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 fish that was in the river died; 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.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.