16:4And the third angel poured out his bowl into the rivers and the springs of waters; and they became blood.
And the THIRD ANGEL poured out his bowl into the RIVERS and the SPRINGS of WATERS;
The second bowl (previous verse) affected the salt water bodies. This one attacks the fresh water. Man uses fresh water for cooking, drinking, and bathing. Now the remaining "666 marked" people are trying to recover from their sores (1st bowl) and are forced to search for fresh water just like the Egyptians did (Exodus 7:20-24).
So complete is this judgment that the reward Jesus talks about in Matthew 10:40-42 is not likely. Note the contrast between "receiving Me" and "receiving the mark of the beast (verse 2)".
Believers in heaven, if they even are aware of earth events, truly can be grateful for the river of the water of life (Revelation 22:1).
and they became BLOOD.
Recall that the two witnesses had this power too. This confirms that the two witnesses were prior to this bowl judgment inasmuch there would be no need to use their power when this bowl takes care of the whole earth. Interestingly, if the Antichrist could see into the future (which he cannot), then the demonstrations of the two witnesses should have convinced him to prepare for this BOWL plague.
From a spiritual standpoint, as 1 John 5:6-12 points out, this conversion between water, blood, and spirit also is a "witness".
Symbolically, scholars who subscribe to the non literal interpretation have a difficult time with this bowl. Some indicate this is a destructive war but this BOWL is global, and a global war here would negate Harmagedon (verses 14-16).
Economic impact: The need for water will cause major riots. As a commodity, pure water produced via technology will be very expensive. This will force most people to drink BLOOD (verse 6) just to survive. It's not clear whether animals balk at drinking blood. If they do, livestock, poultry, and pets are disappearing just as seafood did in verse 3.