A.T. Robertson Commentary Revelation 16:3

"And the second poured out his bowl into the sea; and it became blood as of a dead man; and every living soul died, [even] the things that were in the sea." — Revelation 16:3 (ASV)

Into the sea (εις την θαλασσαν). Like the first Egyptian plague (Exodus 7:12-41) though only the Nile affected then.

Blood as of a dead man (αιμα ως νεκρου). At the trumpet (8:11) the water becomes wormwood. Here ως νεκρου is added to Ex 7:19, "the picture of a murdered man weltering in his blood" (Swete). "Coagulated blood, fatal to animal life" (Moffatt).

Every living soul (πασα ψυχη ζωης). "Every soul of life" (Hebraism, Genesis 1:21, marked by life).

Even the things that were in the sea (τα εν τη θαλασση). "The things in the sea," in apposition with ψυχη. Complete destruction, not partial as in 8:9.