According to my calculations (anyone, please feel free to correct these), it would take 8.8x10^27 cc of water to cover the Earth with 5 miles of water. The volume of the Earth is only 1.1x10^27 cc. Now where did all that water come from? Out of the bowels of the Earth?
Out of a fairy tale?
My calculations were in serious error. It appears it required 10^25 cc of water to flood the Earth - still an incredible number that would require much of the oxygen boung in minerals to be converted to water.
In addition, to flood the Earth in 40 days and 40 nights, it would take 6 inches per minute of rainfall. 12 inches per hour is considered a deluge. (of course there's the minor problem that each inch of rain increases the Earth's diameter requiring even more to keep up the rate).