Having taught the Torah (the Five Books of Moses) from the Hebrew for more than 40 years (hundreds of hours are available by download through my website), I consider the biblical flood story one of the world's most profound moral teachings. As I will show, it means that God cares about goodness more than anything else.
Let me explain by answering the most frequent challenges to the story.
Q: Why did God destroy the world?
A: Because "The Lord saw how great was man's wickedness on earth. ... And the Lord regretted that he had made man on the earth and His heart was saddened" (Genesis 6:5-6).
When God created the world, He announced after each day's creations, "It was good." But only after His final creation -- the human being -- on the sixth day, did God say, "It was very good." God was particularly pleased with, and had the highest hopes for, this creation, the only one created "in His image." This is not about man having God's physical attributes (God is not physical). It is about humans being infinitely more precious than all other creations; and only man, like God, has moral knowledge and therefore moral free will.
When God saw how cruelly human beings treated one another, He decided that He would start over. Once people reach a certain level of widespread evil, life is pointless.
Q: Why did God destroy animals as well?
A: In the biblical worldview, the purpose of all creation is to benefit man. This anthropocentric view of nature, and indeed of the whole universe, is completely at odds with the current secular idealization of nature. This secular view posits that nature has its own intrinsic meaning and purpose, independent of man.
All of creation, in the biblical view, was to ultimately prepare the way for the creation of man. But one does not need the Bible alone to hold this view. A purely scientific reading of the universe is in keeping with this view. Everything -- every natural and physical law -- is exquisitely tuned to produce life, and ultimately man, on earth.
Q: Isn't the biblical flood story just a fairy tale?
A: Two responses:
First, this is so only if you believe that the biblical flood story states that the entire earth from the North Pole to the South Pole was flooded and that every living creature from penguins to polar bears, except for the animals and the people on Noah's ark, was killed. But that is not what the story says. The narrative speaks of the world where Noah lived: It is expressly stated in Genesis 9:10 that there were other animals in the world that were not killed by the flood.