The story of the flood is universal and believable and the idea that God might have warned the one guy to build an ark is also believable. What ISN’T believable is that God would have wiped the entire Earth over sin only to have sin back in business as if nothing had happened forty years later. I give God credit for being a bit brighter than that...
Well, the Bible doesn’t describe Noah’s Flood as an act in order to rid the earth of sin. Throughout the Old and New testament we see the fall of man and ultimately redemption through Christ, and the fulfillment of scripture. The Lord’s flood was a act of judgement.
That’s being over-simplistic, but my point is that the Flood was not to destroy sin in and of itself.