In what way does not believing in the Ark cause the whole thing to unravel?
Frankly, I find the idea that 'God so loved the world he gave his only begotten son to save it,' incompatable with the idea 'God was so disgusted with the world, he drowned everyone except for eight people.'
And another thing: If the entire human race is descended from Noah, around 4,000 years ago wouldn't there only be one single version of the Y-chromosome? Noah's sons would all have his Y. All other human Y chromosomes would have ceased to exist. There would have been one Y and eight X chromosome versions. You would think that would be a testable way to prove (or disprove) the flood, comparing the diversity of the X and Y chromosomes.
ETA: Currently, the Y Chromosomal common ancestor (commonly called 'Adam') is estimated to have lived between 200,000-300,000 years ago. If the flood story is true, we should call that guy 'Noah' instead.