they became blood
In the judgment of the third trumpet, a third of the rivers and springs of water became wormwood. See commentary on Revelation 8:10. The parallels between the second and third trumpet judgments and the second and third bowl judgments are striking. Nevertheless, they are different both in quantity (one-third versus all) and quality (blood versus wormwood). Similarity does not make identity. The bowl judgments do not recapitulate the trumpet judgements.
Overly subtle interpretations in the interest of recapitulation overlook the distinct differences between the two series. Among the more important are: (1) the trumpet-plagues are partial in their effect (one-third of the earth is burned, Rev. Rev. 8:7+; one-third of the sea becomes blood, Rev. Rev. 8:8+; see also Rev. Rev. 8:9-12+) while the bowls are universal (every living soul died, Rev. Rev. 16:3+; every island fled away, Rev. Rev. 16:20+) and final; (2) the trumpets are to a certain extent a call to repentance while the bowls are the pouring out of divine wrath; and (3) man is affected indirectly by the first four trumpets but is directly attacked from the outset by the bowls. It should also be noticed that the bowls are poured out in rapid succession with the customary interlude between the sixth and seventh elements of the sequence missing.1See Literary Structure.
There is a close similarity between this judgment and that which afflicted Egypt prior to the Exodus when the rivers and streams of Egypt were turned to blood (Ex. Ex. 7:20). See The Plagues of Egypt and the Tribulation.
That, then, which has always been the symbol of salvation in the midst of life, becomes the symbol of condemnation in the midst of death. But now, the seas are turned to blood; the fish die; the winds of God blow death over all the earth. They had refused the salvation that would have come to them from the blood of the One who is Life; they now receive condemnation from the blood that symbolizes death.2
The destruction of what is left of the earths fresh water will cause unthinkable hardship and suffering. There will be no water to drink; no clean water to wash the oozing sores caused by the first bowl judgment; no water to bring cooling relief from the scorching heat that the fourth bowl judgment will shortly bring. The scene is so unimaginably horrible that people will wonder how a God of compassion, mercy, and grace could send such a judgment. And so there is a brief interlude in the pouring out of the judgments while an angel speaks in Gods defense.3
3 John MacArthur, Revelation 12-22 : The MacArthur New Testament Commentary (Chicago, IL: Moody Press, 2000), Rev. 16:4.