Not all of this part of the story is inherently unbelievable. But in context with other parts of the story, it is hard to take literally.
13 And God said to Noah, "The end of all flesh has come before Me, for the earth is filled with violence through them; and behold, I will destroy them with the earth. 14 Make yourself an ark of gopherwood; make rooms in the ark, and cover it inside and outside with pitch. 15 And this is how you shall make it: The length of the ark shall be three hundred cubits, its width fifty cubits, and its height thirty cubits. 16 You shall make a window for the ark, and you shall finish it to a cubit from above; and set the door of the ark in its side. You shall make it with lower, second, and third decks. 17 And behold, I Myself am bringing floodwaters on the earth, to destroy from under heaven all flesh in which is the breath of life; everything that is on the earth shall die. 18 But I will establish My covenant with you; and you shall go into the ark--you, your sons, your wife, and your sons' wives with you. 19 And of every living thing of all flesh you shall bring two of every sort into the ark, to keep them alive with you; they shall be male and female. 20 Of the birds after their kind, of animals after their kind, and of every creeping thing of the earth after its kind, two of every kind will come to you to keep them alive. 21 And you shall take for yourself of all food that is eaten, and you shall gather it to yourself; and it shall be food for you and for them." 22 Thus Noah did; according to all that God commanded him, so he did.So, again, I am troubled by every single reference to God being so upset with the violence that He would want to destroy everything. Why does this bother me? Two reasons. First, because it was all as God made it. Our nature comes from God. In fact, He made us in His image. God is certainly quite violent at times, even if you assume that He has no blame in the current violence on Earth, He clearly rained violence upon His enemies throughout the Bible. And, although I can't think of the entire universe of victims, they included the people of Bable, the Egyptian babies, the women and children of Jericho, and so on. And the world rewards violence. God could have made a world in which violence was unrewarding. He didn't.
The instructions for the boat are a bit absurd, too. If you read this the way I was taught to read it, Noah packed in a pair of every living thing on the planet. And everything alive today descends directly from the breeding pairs that were saved by the boat, because everything else alive was to die.
Are there not millions of species of beasts? How big a boat would that have to be? The Queen Elizabeth couldn't do it. Noah's boat was better than anything man can build today. From what I've read outside the Bible, from those who both claim to believe Genesis and claim to know the weights and measures, the ark was about 450 feet long, 75 feet wide, and 45 feet high. And it housed everything from the mice, to the spiders, to the wolves and sheep and elephants. And they all came to Noah's home to be saved. And after they landed, they all went their separate ways again.
When I was young, I loved this story. I loved to envision the pairs of animals, lined up obediently, walking up a ramp onto the boat, two by two. But I can't fathom it now. The carnivores ignoring all these meals on legs? Not likely. But, says my pastor, God inspired the animals. That's is how they knew to come. And that same divine inspiration caused the carnivores to not eat the other animals during the voyage. If so, that would have been the most compelling part of the story, but it goes untold. Besides, it would have made more sense for God to inspire the animals to swim.
And there was "a window for the ark, ... to a cubit from above." I'm not sure what this is supposed to mean, but it sounds to me like God reminding Noah to plug in a single window, for air and light. One window. Millions of animals, eating, and pooping and all that nasty business for several months, and you need one window. Or maybe they didn't have to eat or drink or breathe, because God intervened. But the story doesn't say that, and it would be remarkable enough to mention if He did.
This story only makes sense if you view it as a small boat, enduring a localized flood, with a small collection of local animals and a large stock of food and water. And as such, it would be a great story. But the author of Genesis goes to great pains to make it a flood that covers the entire Earth, killing everyone and everything. And it just doesn't make sense that way.