Noah. No fish were harmed in the telling of this story.

Most ancient cultures have flood stories because most places on earth experienced floods. So people needed to create explanations for the floods that took lives and damaged villages.  It is suggested that the early Hebrews had flood stories but when they were in exile from 587 to 538 B.C.E.  they molded their flood stories into one that followed very closely the Mesopotamian story called the Gilgamesh Epic dated 2500 years B.C.E.  I maintain the story of Noah would have been finalized during the exile period or shortly thereafter. 

We know there is combined sources for the Noah story.  Priestly: sin, punishment, and animals on the ark for sacrifice.   Elohist, Elohim the powerful God of mercy: "the sons of God..." (Gen 6.2 NIV).  "Then the LORD (Yahweh the eternal God of Justice) said, 'my spirit will not contend with men forever'..." Gen 6.3 NIV).  

Don't think to much about Nephilim.  Scholars do not agree on its meaning.  It might be giants which would lend itself to myth stories, or it might mean men that have fallen away from Godly ways, thus, "The daughters of men..." (Gen 6.4b NIV).

The LORD saw how wicked humans had become but Noah found favor with the LORD.  "I have found you righteous in this generation." (Gen 7.1b NIV).  Noah was not pure or perfect but was evidently the most righteous of his generation.  Chapter 6.9 has Elohim the powerful God of Mercy speaks to Noah.  In chapter 6.19 Noah is instructed to take two of every kind of animal and bird into the ark. 

When chapter 7 starts we have a priestly source using the name Yahweh the eternal God of Justice to take 7 of every kind of clean animal and two of every kind of unclean animal.   Some of the 7 clean animals and birds would be sacrificed.  The difference in the instructions in chapter 6 and the beginning of chapter 7 is an example of how the differing sources are co-mingled by later editors forming the Old Testament and that dovetailing of sources makes the Bible a dickens to understand for those of us reading it 2500 years after it was put together.  

When the flood was over Noah built an altar to the LORD and sacrificed some of the clean animals.  So you ask, why bring some unclean animals?  I suppose by the time this was written after the exile there were by Jewish standards, unclean animals on the earth so there had to be a reason unclean animals survived the flood.  Don't give it to much thought.  

Chapter 9 brings the story to a climax with the Noahide Laws.  Seven laws that make up a covenant between God and all the humans on the earth that are descended from Noah.  It is said that non-Jews that keep these laws can be in a relationship with God and have access to the afterlife.  The laws in chapter 9.1-7: a prohibition against (1) idolatry, (2) murder, (3) theft, (4) sexual immorality, (5) blasphemy, (6) eating flesh from an animal while it is still alive, (7) the requirement to have laws and courts and establish legal recourse.  

Then Noah planted grapes and made wine.  He was over 600 years old and was not accustomed to wine and became intoxicated.  What happened next makes no sense and scholars debate the details and there is no consensus on why or what it means.  Ham, the youngest son saw his father drunk and naked.  That can mean what it says therefore to see your parent naked was sinful.  Or it could mean he had sexual relations with his passed out father.  We just do not know what it means.  When Noah awoke he cursed Ham's son Canaan.  Why not Ham?  Perhaps is is the cursing of the decedents of Ham.   Sadly Christians throughout early history used these verses to justify slavery.