The second angel poured out his bowl into the sea, and it became like the blood of a corpse, and every living thing that was in the sea died.
In Rev. 8, we see this as something like a mountain being thrown into the sea.
An undersea volcano was the best fit. The island volcano has exploded and killed one-third of the fish. Now the eruption continues and pours chemicals into the sea, making it so toxic that everything dies in the sea that the mountain is thrown into. This could not happen quickly, as the toxic substance would take a while to travel the sea if it were one of the oceans. However, if it were a small sea, it could be quick. Remember, it is not all seas, but the sea the mountain is thrown into. Do you see how the trumpet tells us how this begins, and the wrath tells us how it ends?