Nothing in Genesis 6ff would lead us to think it is a parable. Jesus does not identify it as a parable.
I spent this fall in pretty consistent study of the flood, vertical plate tectonics, geo-mythology, and the literary question of whether basic world myths came together in the Bible or descended from it. The Genesis flood is in a sense easier to demonstrate than creation is. Part of that is chronological; as you investigate residue evidence, you come to indicators of the flood before you do creation, although you might say creation is the evidence that is hidden in plain sight, right?
There is simply no question at all that there has been a massive deluge, but please scrap all 'sunday school' imagery before you look into it. It is only about rain in a very secondary sense. It has much more to do with vertical tectonics, and even the atheist Ager in NEW CATASTROPHISM says that our surface is much more convulsed than most people are willing to admit.
Please notice something the next time you look at Monument Valley near the 4 Corners of the US: what you are seeing is the remnant of material AFTER massive flooding and slurry. How, hydrologically, do you move material like that? And since some of it is from New England, what speeds must you reach to get hundreds of feet layers from New England to the SW as slurry? Just that snapshot should help you realize how much and how enormous the event was. It is matched by the 'Centralia' theory in Australia: the entire center is now believed to be a rapid sediment deposit that had so much force that it 'bent' Ayers Rock into its J shape, the bottom of which is the outcrop we see today.
You could also check my short novel, DELUGE OF SUSPICIONS.