When God revealed His plan to destroy the world with a Flood, He told Noah, “And the Lord said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them.” Genesis 6:7
Fish and sea creatures were not mentioned. Several passages in the Flood section of Genesis help shed some light on the question of whether or not fish were killed in the Flood. “And, behold, I, even I, do bring a flood of waters upon the earth, to destroy all flesh, wherein is the breath of life, from under heaven; and every thing that is in the earth shall die.” Genesis 6:17
Specifically, everything on the earth would die. The flood of water would drown them. Logically, animals that live in the water would not be affected by more water. Also, it is hard to see how fish could be described as having “breath.” Land animals and birds were selected to board the ark, but fish and sea creatures were not (Genesis 6:19-21; 7:2-4). This would indicate that sea creatures did not need the ark in order to survive.
Genesis 7:20-23 lists animals that died, but sea life is not included: “Fifteen cubits upward did the waters prevail; and the mountains were covered. And all flesh died that moved upon the earth, both of fowl, and of cattle, and of beast, and of every creeping thing that creepeth upon the earth, and every man: All in whose nostrils was the breath of life, of all that was in the dry land, died. And every living substance was destroyed which was upon the face of the ground, both man, and cattle, and the creeping things, and the fowl of the heaven; and they were destroyed from the earth: and Noah only remained alive, and they that were with him in the ark.” Genesis 7:20-23 The reference to “dry land” limits what types of animals were affected.
After the Flood.. “And God remembered Noah, and every living thing, and all the cattle that was with him in the ark: and God made a wind to pass over the earth, and the waters assuaged;” Genesis 8:1.
Why didn’t God “remember” the fish and sea creatures? They had not been destroyed like the other animals. The fifth day of creation included the creation of fish and sea life as well as flying creatures and birds: “And God created great whales, and every living creature that moveth, which the waters brought forth abundantly, after their kind, and every winged fowl after his kind: and God saw that it was good.” Genesis 1:21
Land animals were created on the sixth day (Genesis 1:24). Based on these verses, the Jewish people saw animal life in three categories—sea life, flying animals, and land animals. Of those three categories, only the flying animals and land animals were taken aboard the ark. The fish and sea life were not at risk under the water.