point number one
This seems obvious enough
The Hebrew says that God will destroy man from the face of the earth. The word eretz earth is land. Aquatic life is not on the land. Not on the face, or surface of the land.
The means of destruction was via water. It’s extremely difficult to drown a fish or aquatic life.
The OP’s English translation says “and animals”. But the Hebrew says and behemah or beast. One could use this to argue that fish aren’t beast. This isn’t the strongest argument because behemah could be used to refer to animals collectively. But it’s specific to beasts on the (sur)face of the earth, which again aquatic life is in the water.
“And all flesh died that moved on על the earth, birds, livestock, beasts, all swarming creatures that swarm on על the earth, and all mankind. Everything on the dry land in whose nostrils was the breath of life died. He blotted out every living thing that was on על the face of the ground, man and animals and creeping things and birds of the heavens. They were blotted out from the earth. Only Noah was left, and those who were with him in the ark.”
The Bible goes through the effort to highlighting that these were upon the earth, not in the water.
Why were the aquatic life spared? By chance they escaped. But in the final destruction, everything will be destroyed, heavens (full of water) and the earth too. They will all pass away and there will be a new Heaven and a new earth. Nothing will escape that dreadful day of the Lord
“But the day of the Lord will come like a thief, and then the heavens will pass away with a roar, and the heavenly bodies will be burned up and dissolved, and the earth and the works that are done on it will be exposed.”
2 Peter 3:10
There’s no escaping this fire