God said right up front that the reason he wanted to punish mankind was for wickedness. So his plan was to flood the entire world (however you define the word 'world'.)
Isn't that overkill? Why kill animals? Why destroy entire ecosystems? Isn't that like ridding your house of carpenter ants by setting the whole house on fire?
Why destroy children in their mother's wombs? Were they hopelessly wicked too? It took Noah up to 100 years to build the ark--surely not every person that hadn't even been conceived yet when God laid out his plans to Noah would have been so wicked as to deserve the death penalty.
Is this really the best that God could do? Where was the influence of the Holy Spirit that Christians talk about, the one that draws hearts to God? Why didn't God incarnate and show mankind how to be a righteous person?
Inerrantists (Hey, I used to be one) bend themselves into knots to explain how kangaroos got back to Australia after the flood, or how eight Middle-Eastern people evolved into the many racial varieties we see today in only 100 generations, but they can't explain how in the world such a horrific genocide could be the well-reasoned and best plan from a God who "so loved the world." Inerrantists can wave away criticisms all they want about genotypes and "vegetation mats". But if we take this account at face value (and inerrantists always insist that we should) then God is a moral monster.
Look, every time there's an earthquake or hurricane, some preacher says it happened because God is punishing someone for something. The 'who' and the 'for what' change over time, naturally, but it always comes down to God using a big hammer to smash a tiny bug. Why wouldn't Noah's flood be the same? We're a species with a propensity to live on shorelines on a world that's three-fourths covered with water. Flooding is by nature a common occurrence. So way back when, a flood caused a huge hullabaloo, and someone said, "God was punishing those wicked people that I don't like." And like most more-or-less decent stories, it took hold in the human mind and has hung on for dear life in the face of counter-evidence.
Inerrantists have to defend the Genesis flood because they know a camel's nose in the tent when they see one. If we chuckle to ourselves and set aside Noah's Flood as nothing more than an amusing story, then we open the door to other possibilities. What other amazing stories in the Bible can set aside as nonsense--the Ten Plagues of Egypt? The stopping of the sun in the sky--twice? The Virgin Birth? Maybe...the Resurrection? But from there Christianity won't have much to support it, and inerrantists will either have to find other work, or they'll have to swallow hard and admit that they were wrong.
Having done exactly the second option, I can confidently say that it ain't easy.