The second vial is poured out upon the sea. And it became as the blood of a dead. And every living being, that was there in the sea, died. 

The sea is a symbol for the life forces, the etheric body of man and earth. Blood is in the human body the carrier of life, but outside the human body it becomes a sign of death. All living and animated creatures in the sea die. Once life started in the sea, now life ends here.      

The end of astral fire

During the cycle of the seals, the world still existed of four parts: the physical, the etheric, the astral and the spiritual spheres. During the cycle of the trumpets the world knew three spheres: the etheric, the astral and the spiritual spheres of which at the end the etheric sphere disappeared. During the cycle of the vials of wrath all life ends. The astral sphere is withdrawn in the spiritual sphere. The astral fire of the passion that lives in our blood, digests all remaining life in the etheric sphere and finally disappears itself. (Schult, p.268).

The outpouring of the vials of wrath, Matthias Gerung, Ottheinrich-Bible, Bayerische Staatsbibliothek, 1530-1532,  https://en.wikipedia.org/wiki/Seven_bowls#/media/File:Ottheinrich_Folio298r_Rev16A.jpg

The increasing intensity of events

We can find clear parallels of the second vial and  the second trumpet and the second seal. At the blowing of the second trumpet a waving flood of fire, great as a burning mountain, was cast in the sea and a third part of the sea became blood and the third part of the creatures that were in the sea, and had breath, and the third part of the ships, perished (text fragment 23). At the opening of the second seal (text fragment 14) a horse comes forward, fiery red, and to him that set thereon power was given to take peace from the earth and a great sword was given to him. These fragments illustrate the intensification of effects in the course of the Apocalypse from the effects of a sword, of a flood of fire that kills a third of living creatures in the sea, till here, where all life in the sea perishes, because all material life ends.