Then the third poured out his ceremonial bowl into the rivers and the springs of the waters, and it became blood.
Then I heard the angel of the waters saying, “You are righteous, One who is and who was, the Holy, because You judged these things, and because blood of holy ones and of prophets they poured out, and blood You have given to them to drink: they are worthy.”
Then I heard I heard the altar of incense saying, “Truly, Lord, God, the omnipotent, true and righteous are Your judgments.”
Revelation 16:4-7

What you just read is how this passage is written in Greek with a very few modifications in how the words are placed in the sentence, just to give a nod to English grammar (which is a bit different than Greek grammar).
“The third” is the third angel (who else could it be?) once the passion contained in the ceremonial golden bowl was poured out into all the running water of the earth, the water turned to blood. This is much more like the ancient Nile plague, for this is flowing blood.
The “angel of the waters” is a mysterious figure. It seems the rivers and springs have presence of some kind, and that presence acknowledged to God the righteousness of God’s judgment. There is a symmetry here. The dragon and its savage creatures had shed copious volumes of blood—the blood of holy ones, the Lord’s own, and of the blood of prophets (namely, the two prophets sent to them in warning). Now, God would give them, and the irredeemable remnant left on earth, only rivers of blood to drink from.
Just as the infant Hebrew boys who had been thrown into the Nile at Pharaoh’s command were worthy of the first plague God leveled against Egypt, so also now, the martyrs are worthy of this act of God.
“The voice of the altar.” In fact, from John’s vantage, it seemed the altar of incense, standing before God in the sanctuary of God’s heavenly throne room, also gave voice in affirming God’s judgement. But in this case, surely these were the voices of the martyrs themselves, who had been given white robes and assurance that God would, indeed, vindicate them at the appointed time.
Historicist
Likening the third bowl to the third trumpet, scholars look to the region of Europe where river and streams stem from, where Attila the Hun was identified as the fulfillment of the third trumpet. The bowl is fulfilled in the same region, this time in the French Revolution, where bowls one through four coincide.
So, just as blood poured forth through all the rivers and rills – picture the spread of red from a central place, now spidering outwards – so the French Revolution spread in the form of invasions as Napoleon sought to colonize all Europe as his empire. Many of the battles were along the shores of the Rhine, which flows through western to eastern Europe, and along the Danube and Po, as well as all the springs that stream through the Alps.
Incidentally, all the regions invaded by France were also a part of the Roman Church’s territory, marking infamous areas of Protestant clashes with Catholic authorities, resulting in the martyrdom of many. In this way, scholars see the third bowl clearly fulfilled, for the blood of the Protestant martyrs and those who spoke with a prophetic voice, is vindicated by God in the French Revolution’s destruction of the Roman Church’s monopoly on power and ecclesial rule.

Preterist
Though the bowl may pertain to an event during the long slow demise of the Roman Empire, there is also evidence for a specific fulfillment in the Jewish War of 66-70 CE.
“… not only the whole country through which they fled was filled with slaughter, and Jordan could not be passed over, by reason of the dead bodies that were in it, but because the lake Asphaltites was also full of dead bodies, that were carried down into it by the river. “
Flavius Josephus, THE WARS OF THE JEWS, Book IV, 7:6 (Translated by William Whiston)
Choked with the bodies of the slain, pollution was the natural consequence, so that the rivers and streams literally became undrinkable because of shed blood.
Jerusalem itself was also contaminated by the putrefaction of blood literally streaming down streets and alleyways from infighting between factions, and finally the brutal slaughter leveled by Roman troops.
Water, as one historicist commentator points out, is a symbol of life, of God’s blessing and goodness, all throughout the Bible. The Garden of Eden, paradise on earth, is described as having four mighty rivers running through it. When Jesus spoke of living, flowing water, He meant the very life and Spirit of God.
“… and let the one who believes in me drink. As the scripture has said, ‘Out of the believer’s heart shall flow rivers of living water.’ ” Now he said this about the Spirit, which believers in him were to receive …”
John 7:38-39 (NRSVUE, italics added)
When this third bowl is poured out, the blessing is turned into its opposite, a curse spreading throughout the land, a macabre picture of the life of God polluted by the shedding of holy blood. An image of the cross looms, for indeed, the Holy blood of Christ was shed, a river of life poured out for the many.
Those who are given this blood to drink are the ones who shed the blood of the holy ones, and of the prophets. To preterist scholars, these can be none other than the Sanhedrinists, and all those who stood with them, calling for Jesus’s execution.

Jesus indicted these religious rulers by saying,
Woe to you, scribes and Pharisees, hypocrites!
For you build the tombs of the prophets and decorate the graves of the righteous, and you say, ‘If we had lived in the days of our ancestors, we would not have taken part with them in shedding the blood of the prophets.’
Thus you testify against yourselves that you are descendants of those who murdered the prophets. Fill up, then, the measure of your ancestors.
You snakes, you brood of vipers! How can you escape the judgment of hell? For this reason I send you prophets, sages, and scribes, some of whom you will kill and crucify, and some you will flog in your synagogues and pursue from town to town, so that upon you may come all the righteous blood shed on earth, from the blood of righteous Abel to the blood of Zechariah son of Barachiah, whom you murdered between the sanctuary and the altar. Truly I tell you, all this will come upon this generation.
Matthew 23:29-36 (NRSVUE, emphases added)
And in point of fact, these very same authorities willingly took upon themselves the disaster Jesus spoke of, crying, “His [Jesus’s] blood be on us and on our children” (Matthew 27:25)! With those words, the religious rulers and the people with them called upon themselves the third bowl of the passion of God’s wrath.
Futurist
There is a progression from a third of freshwater turned bitter by the third trumpet, to all freshwater become blood from the third bowl. If water does symbolize life and goodness, then God’s judgments in these forms reveal them as now corrupted and poisoned by earth’s sin.
But, it could also be a literal revelation of the dragon and its craven creatures’ lust for blood. Now, they shall have all the blood they could possibly want, to slake their thirst.

Spiritual
God’s judgment is equal to the crimes committed—the butchery of God’s beloved and the massacre of God’s sent ones the prophets must now be balanced by a comparable flow of blood. The lifeblood of earth itself is water, and so now earth is made to bleed, for that is what all the killing is about. God’s judgments are just.
The altar also speaks because it is the altar of incense, from which rises the prayers of the people asking God for justice. God sent a severe warning in the trumpets. For those who did not repent, the bowls will be God’s judgement.

The four perspectives taken from Revelation: Four Views A Parallel Commentary, edited by Steve Gregg