Revelation 16:3

3. The second bowl 16:3

This judgment resulted in the destruction of all sea life, not just one-third of it, as in the second trumpet judgment (Revelation 8:8-9). This may involve actual coagulated, rotting blood or a liquid that resembles blood (cf. Exodus 7:19-25). I think it probably means real blood. Some commentators interpret the sea as the masses of humanity, but there are no clues in the text that this sea is any more symbolic than the waters that Moses turned to blood.

"The sea is a great reservoir of life. It is teeming with life, and the salty water is a cathartic for the filth of the earth. However, in this plague, blood is the token of death; the sea becomes a grave of death instead of a womb of life." [Note: McGee, 5:1023.]