“They have power to turn the waters to blood” is part of the description of the two witnesses [Rev. 11:6]. In this article, the two witnesses are understood to represent the Scriptures, and the Spirit, the two things that Jesus said testify of him. [John 5:39; 15:26]
The waters in the prophecy signify the prophetic Scriptures. Blood signifies something that contaminates, that men cannot eat, or drink. Blood was forbidden to all men in Genesis. God said, “But flesh with the life thereof, which is the blood thereof, shall ye not eat.” [Gen. 9:4] It was forbidden to Israel in the law of Moses. “Ye shall not eat any thing with the blood.” [Lev. 19:26]
In his article on The Seven Trumpets Of Revelation, Henry Bechthold proposed a unique interpretation of the cavalry of 200 million horses and horsemen of the second woe. He claims that both the horses and horsemen of the sixth trumpet, and the two witnesses, represent the church, and God’s end-time servants.
Gregory K. Beale is Professor of New Testament and Biblical Theology at Westminster Theological Seminary. In his commentary on Revelation he says the two witnesses of chapter 11 represent the whole church, not two individual humans. Beale wrote: 
Elements of the prophecy of the two witnesses of Revelation 11, possibly alluding to the account of Adam and Eve in the garden of Eden in Genesis 2 & 3 are identified in the table below.
William Milligan (1821-1892) was Professor of Divinity and Biblical Criticism in the University of Aberdeen. The following is his commentary on the prophecy of the two witnesses of Revelation 11. [William Milligan. The Book of Revelation. In: Marcus Dods, Robert Alexander Watson, Frederic William Farrar, eds. An Exposition of the Bible: a series of expositions covering all the books of the Old and New Testament, Volume 6. S. S. Scranton Co. Hartford, Conn. 1904. pp. 873-879]
The word of God in prophecy is like mountains, and the theories, and opinions, and interpretations of men are like clouds in comparison. The prophet Joel spoke of the day of the Lord as “A day of darkness and of gloominess, a day of clouds and of thick darkness.” [Joel 2:2] Clouds are associated with darkness of a spiritual kind.
Peter described the false teachers as “clouds that are carried with a tempest; to whom the mist of darkness is reserved for ever.” [2 Peter 2:17]
The sceptical Westminster Review for Oct. 1861 included a “secular exposition” of the Apocalypse, based on the work of various critics.
The author or authors of the essay took some pains to promote the notion that the Apocalypse was written before 70 A.D. Attempts at dating the writing of the Apocalypse before 70 A.D. are founded upon the idea that the cities mentioned in Rev. 11, the ‘holy city’ of vs. 2, and the ‘great city,’ called ‘Sodom and Egypt’ in vs. 8, are the same. This they insisted upon, while defending the opposite view of the temple of God mentioned in vs. 1 and vs. 19. These temples of God, they claimed, are different; the temple of God in Rev. 11:1 is located in the earthly Jerusalem, that (in their opinion) was not yet destroyed; John had to descend from heaven to measure it; but the one in vs. 19 is the temple in heaven.