I am sorry to see your error stated as fact. Please reread this passage and tell me where you get anything about sea creatures... I can't find it, nor could anybody in my Bible College class on Genesis!
Genesis 1:21 Every living thing that moved on land perished—birds, livestock, wild animals, all the creatures that swarm over the earth, and all mankind. 22 Everything on dry land that had the breath of life in its nostrils died. 23 Every living thing on the face of the earth was wiped out; people and animals and the creatures that move along the ground and the birds were wiped from the earth. Only Noah was left, and those with him in the ark
Get your self a real Bible:
Genesis 6:17
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.
OK? - All flesh - everything that is in the Earth shall die.
There were no exceptions, all animate life was corrupted and had to die.