Revelation 16:3 The second angel poured out his bowl on the sea, and it turned into blood like that of a dead person, and every living thing in the sea died.

Well, just restates the verse.

The 2nd Plague—The Sea Turns to Blood: This second plague kills every living thing in many of the seas and oceans of the world.

The site offers a little more.

The sea… became blood: Revelation 8:8-9 described a partial contamination of the sea. Here the contamination is made complete (every living creature in the sea died).

Blood as of a dead man: The sea doesn’t necessarily become blood, but as of a corpse’s blood. It will match the appearance and sickening character of the blood in a dead body.

This is from

1. The second angel poured out his bowl into the sea = moves from man to nature. The whole of salt-water sources is the object of the second bowl.

2. It became blood like that of a dead man = is the impact of the second bowl. A figure of speech is employed. The blood of a dead man is no longer capable of supporting life. Blood is the aspect of the dead that cannot be used for transplant or transfusion purposes.

3. Every living thing in the sea died = is the outcome of the bowl.

IMHO.... humans are already working on turning the oceans into sludge. We're such messy creatures.

The sea is necessary for life. The sea is made of water... that water evaporates.... then the vapor is carried over land where it is deposited as rain... then the rain water gathers and runs to the sea and it starts all over again. That water is made of oxygen and hydrogen.... the earth thrives on oxygen.

If the oxygen is depleted.... then air is depleted.... plant life, animal life, humans... we all depend on air.... if the sea goes dead....

This is a devastating blow. As the men on "Dual Survivor" would say.... "without water we're dead".