It is easy when looking at prophecy to think that all the water is just that water. However, the Bible tells us what the water is representing.

And he said to me, “The waters that you saw, where the harlot is seated, are peoples and multitudes and nations and tongues. Rev. 17:15 

Now we can take the definition and go back over the prophecies and see that the beastial nations come out of the peoples, nations, and tongues.

. . . therefore, behold, the Lord is bringing up against them the waters of the River, mighty and many, the king of Assyria and all his glory; and it will rise over all its channels and go over all its banks; Is. 8:7 

And four great beasts came up out of the sea, different from one another. Dan. 7:3 

And I saw a beast rising out of the sea, with ten horns and seven heads, with ten diadems upon its horns and a blasphemous name upon its heads. Rev. 13:1 

From this, we conclude that local bodies of water mean the nation that it is in. So when the Nile dried up the power of Egypt waned, as when the Euphrates dried up the Ottoman Empire was at its end.

They shall pass through the sea of Egypt, and the waves of the sea shall be smitten, and all the depths of the Nile dried up. The pride of Assyria shall be laid low, and the scepter of Egypt shall depart. Zech. 10:11

The sixth angel poured his bowl on the great river Euphrates, and its water was dried up, to prepare the way for the kings from the east. Rev. 16:12