Rather than kill everyone with a click of the fingers, God decides to torture people by drowning them. On top of that he can't be bothered to save the only innocent people on the planet - he tells Noah to save himself by building a ship. Another click of the fingers, Noah, his family and the animals could have been teleported into the future - surviving/missing the entire ordeal. But no...oh no!... God has to work in mysterious, woefully inefficient and cruel ways.

During the flooding, several things would have happened: All under water plant life would have died as sun light would have been
blocked out by the rise in sea level. All fish dependent on the underwater kelp would have died as their main food source shrivelled away.
All carnivorous fish dependent on the above fish would have died in a similar fashion.
All plant life and fresh water life would have died as their water source would have been contaminated by the salty flood waters.
All terrestrial life would have drowned - as we know.

So not only do 99.9% of humans die but the ecosystem has to be, for a lack of a better word, F&*# in the process. Talk about overkill. Without an ecosystem to sustain them, Noah and the animals would have starved to death after eating each other. How in the world does this story benefit mankind?