The reader may recall that these are not the first of the book's judgments to affect the earth's waters. We are reminded of the repetitious and cyclical nature of John's judgment symbols. Given the necessity of water for human survival it is not surprising to see it used as the basis of apocalyptic symbolism.
Water is essential to life.
This fact is so readily recognized by humans that God used the element of water to represent eternal life itself. These reserves, however, would not last long. It was the very next angel whose bowl would contaminate the fresh waters of the rivers and springs. With this accomplished, death is near. The identity of these people is no mystery.