There are two ways to interpret the great flood; literally or symbolically. Literally, scientists have found an ocean of water, the size of the Arctic Ocean, in the earth's mantle, below southeast Asia. Beyond that pocket of water there is a lot of water below the earth's crust, under high pressure.
Another group of scientists found a huge scar, on the bottom of the Atlantic ocean, where the mantle is exposed. If we put these two observations together; one might speculate that a great flood could have been cause by a large pool of mantle water, below the Atlantic ocean, breaching and eroding the crust. The high pressure mantle water would force its way into the Atlantic ocean and into atmosphere; warm the ocean and atmosphere. There would have been huge clouds and tidal waves, with the entire earth impacted by the imbalance. The receding flood water might be the hydraulics sending water back to the mantle where the crust is subverting.
http://www.astrobio.net/news-exclusive/scientists-detect-evidence-oceans-worth-water-earths-mantle
Symbolically, a great flood would be connected to the unconscious mind overwhelming the ego. Humans have two centers of consciousness; ego and inner self. A great flood would be symbolic of the inner self and unconscious overwhelming the ego; for a system upgrade. The symbolism of two of each animal being gathered has a connection to the gathering all the natural male and female instincts for the upgrade.
A symbolic flood would have suggested that humans, through willpower and choice, had become unnatural to the point it was damaging the natural control systems of the brain and DNA; sickness and compulsions. This led to an unconscious reaction to restore balance. Noah senses the impending flood in advance, and begins to prepare. After the great flood, civilizations advance; ego 2.0.
A small scale analogy for the symbolism might be a young person experimenting with drugs. At first, their willpower and choice seems to be rewarded, with the excitement of discovery and new experiences. But as this behavior persists, it starts to lead to a routine and addiction; all the lessons have already been learned. Unconscious potential builds since they are willfully shifting the personality and the physical body away from he natural set point. The cleansing affect may be a flood of emotions; rage and sadness, leading to rock bottom; reborn from death. Now the young person has a new perspective based on the wisdom of experience. This impacts them deeply; inner self.
A world wide flood, in the symbolic sense, would sort of be proof for evolution from a common genetic base. Parallel myths in separated cultures would imply a coordination of human advancement, where the same change is occurring in all places; global system upgrade.
It would imply something like Microsoft or Apple announcing the next operating system release months in the future. Noah had access to a beta version. This has bugs, so nobody wishes to use it just yet. But even with bugs, it gave Noah time to prepare for the change and get used to it. Noah stresses before everyone else, for decades. When the new OS is finally released; flood, all the people are confused and unprepared. Everyone but Noah so used to the old interface that the new OS screws up all that you do. Religions are the caretakers of the operating system and help prepare humans for system upgrades.