I was originally taught that the plagues of the vials are upon the entire earth, such that:

Revelation 16:3 And the second angel poured out his vial upon the sea; and it became as the blood of a dead man: and every living soul died in the sea.

Meant that all the oceans of the earth became as the blood of a dead man in the final days of the great tribulation.

Yet, two chapters later, it mentions:

Revelation 18:17 For in one hour so great riches is come to nought. And every shipmaster, and all the company in ships, and sailors, and as many as trade by sea, stood afar off,

  18 And cried when they saw the smoke of her burning, saying, What city is like unto this great city!

  19 And they cast dust on their heads, and cried, weeping and wailing, saying, Alas, alas, that great city, wherein were made rich all that had ships in the sea by reason of her costliness! for in one hour is she made desolate.

It says every shipmaster - and all the company in ships, and sailers - and as many as trade by sea.

Are they still out there, at sea, on the oceans that became as the blood of a dead man, still trading with Mystery, Babylon?

Or is the sea upon which the second vial is poured out, simply, as it says, a single sea, like the Mediterranean, Black, Caspian, or Galilee?

But the waters about Mystery, Babylon, at the same time, are not as the blood of a dead man, and are still filled with ships, still trading?

Where their owners are still made rich by trading with Mystery, Babylon, up to the last minute?