This can be stated in active form. AT: "the angel threw something like a great mountain burning with fire" (See: [[rc://en/ta/man/translate/figs-activepassive]])
The fraction "A third" can be explained in translation. AT: "It was like the sea was divided into three parts, and one of those parts became blood" (See: [[rc://en/ta/man/translate/translate-fraction]])
# became blood
Possible meanings are it 1) "became red like blood" or it 2) really became blood. (See: [[rc://en/ta/man/translate/figs-simile]])
# the living creatures in the sea
"the things living in the sea" or "the fish and other animals that lived in the sea"