When we read Revelation 16: 3, we have the second cup, and that cup made the waters turn to blood, and the animals died. And how it could happen. This occurs through an algae growth factor, called the red tide. This red tide produces several toxins, which kill marine life. And so marine animals are killed.
With the large production of industrial products for agricultural production, such as nitrates and phosphates, which are launched on large-scale plantations. These nitrates and phosphates end up falling into the sea water, and these algae become unbalanced and feed on such products, thus producing toxins, and making the waters acidic, and reducing the oxygen in the water. This results in the high mortality of fish.
But wasn't the verse talking about all the seas? The verse speaks of the sea, and speaks of the animals in the sea that have turned red. So every animal that was in the sea that turned red died.
If we read in Exodus 7:21, it speaks of the fish that died because the river became blood. The Nile River that we define today, starts in the Mediterranean Sea, and goes to Tanzania. Do you mean that in Moses' day, the whole Nile River became blood, or in the region they were in?
This is a Biblical principle that we must understand. That an effect can occur in a region, and that region is determined as the whole as a whole. For example, when we read the book of Acts 24: 5 we have that the text says that the Jews were dispersed throughout the world. We are talking about Rome, Rome was unaware, for example, of the population of China, and was unaware of American Indians such as the Mayans and Aztecs. Did the Jews go to America in Roman times, and become Mayans? Of course not.
So the term everyone, or all, or none, are terms that must be analyzed carefully. Thus the fish and the sea in which it became red, such animals died. So we have the definition of the second cup. The sea that turns into blood and the sea animals that die.
Many people want miraculous things to happen, but that is not necessary. God made a covenant with Noah, as we see in the text of Genesis 9:13, in which God places a bow, which is the rainbow, as a covenant that he would no longer destroy the earth with water. But the rainbow is the refraction of white light, and that moisture does that refraction. So the rainbow is not a miraculous system, but it is a pact factor with Noah. So both the first cup and the second cup do not need miraculous effects. And if people expect miraculous things, they may not be watching the sign of the end times.
Remember that the birth of Jesus would be the great change in the destiny of the earth, and Jesus was born in a manger, and only a few who sought the event and, not miraculous scenes, managed to find Jesus.