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.
4 The third angel poured out his bowl on the rivers and springs of water, and they became blood. (Revelation 16:3-4)
Now as the angels pour out the second and third bowls upon the land we see that all the water in the world, or at least all the bodies of water, become blood. Note that this curse is much worse than the trumpet judgments that sounded earlier in the tribulation period (see Revelation 7:8-11). In that case only a third of the waters were destroyed, allowing many people to survive. However, in this case there is no hope for survival apart from a miracle. The doom of the planet has been sealed. How can the world survive this catastrophe? The answer, I believe, is that Jesus will intervene immediately after returning to Earth in order to begin revitalizing the planet beginning with Israel, as is indicated in the Old Testament prophets (Ezekiel 47:1-12, Amos 3:18). This healing must begin very soon after this plague, or else no one on the Earth would survive. That is why I believe that these bowl plagues take place only a couple days or so before Christ returns.