So what? When you're in the miracle business, what difference does that make? In for a dime, in for a dollar.
1) Displacement of ocean waters could cover the inhabited world and it could take a year for the water to drain back into the ocean, especially if the ship was over flat ground like Mesopotamia. You don't need twice as much water, you just move the water you have over the land. This could come from many sources, including well-placed meteorites or bubbling up of the compressed methane on the ocean floor.
2) The plate boundaries of the Earth contain oceans full of water, if ever they "belched" it back up, their WOULD be obout double the water in the oceans. I don't know of any mechanism that might do this, but the account does say that 'the fountains of the great deep were broken up" in addition to the 40 days of rain.
I DON'T Think it happened 4,000 years ago, or even 7,500, but I don't think a world-wide (as they knew it) flood catastrophe that almost extincted the human race can be ruled out.