Thinking back to my grade school science classes I remember something called the water cycle. Water is evaporated, then condensation leads to precipitation which is either absorbed into the ground or runs off into the sea or other body of water, and the whole process starts all over again. The long and short of it is that it's pretty much a closed cycle. We're not making any more water, just reusing the water we have on the planet currently. So if we accept that then where did the additional moisture come from to cause enough rain to cover the earth to a depth of at least 15000 feet. And where did all that moisture go afterwards?
The water canopy theory accounts for the extra water, but I couldn't tell you where the water went afterwards.
What if the tail of a comet, composed entirely of ice, were to pass close to the earth? Wouldn't that create a huge rainstorm? I think the Bible mentions the waters under or within the earth, and we do know about ground water and artesian wells. Couldn't a lot of water flowed under the earth's crust if a large earthquake created a deep crack?
Just a couple of possibilities.