What does Revelation 16:3 mean?

The second angel empties his bowl into the sea and the sea becomes a putrefying mass. This verse describes the sea as "like the blood of a corpse." This is an unusually specific reference, meaning the writer has in mind a body that's been dead for some time. Whatever happens to the water, it becomes like the rotting fluids found in a decomposing body. As a result, everything in the sea dies.

When God struck ancient Egypt with plagues, He turned the Nile into blood (Exodus 7:19–21), but this second bowl judgment is more widespread; the language used here suggests it might pollute all oceanic bodies of water. Further, the second bowl judgment is far more intense than the second trumpet judgment that turned one third of the sea into blood and killed one third of marine life (Revelation 8:8–9). It seems the trumpet judgments expand the seal judgments, and the bowl judgments expand the trumpet judgments. Each series of judgments is more intense than the previous series. Everything is moving toward a climax, when Jesus returns to earth to capture the beast and the false prophet and cast them into the lake of fire.

No doubt, this judgment cripples the beast's shipping and fishing industries. The effects of this judgment would be even further reaching than simple commerce. A massive change in ocean waters can have a drastic effect on weather and rain. Also, if "everything" living in the oceans were to die, it would remove a primary food source for the entire world. Even more dire, ocean plants produce most of the oxygen in the atmosphere. The death of everything in the oceans would start a countdown to the extinction of all life on earth.