Let's not forget that this global flood was a divine act. There is no need to prove that it would be naturally possible. There does not need to be sufficient water present now to cover the earth, for God to cause a global flood. (God can say let the Earth be flooded with buttermilk and the world would be flooded with buttermilk. God could then say milk be gone, and then it would be gone.)
Also let's not assume that the earth today is the same as the earth was back then. It's my understanding that there is a balance to the earth's crust. A high mountain range is balanced by a deep ocean valley. It is possible that the the earth back then consisted of smaller mountains and much shallower oceans. The pro-flood position is that the flood radically altered the Earth. (I am being extremely brief, so please cut me some slack.)
And God could have used allegories. But fundamentalists don't like that one. Many religions have a birth (Creation story in the Christian faith's case) and a rebirth (the flood wipes it out and voila--new world) story. Just following the typical pattern of religious stories.