We’re not all underwater now. So something happened to the water after the flood. What was that?
Creationists don’t have a coherent explanation for this. Some creationists say that Psalm 104:6-8 suggests an answer. After the waters covered the mountains, God rebuked them and they fled. Then the mountains rose, the valleys sank down. Effectively they claim that the waters are still here! The terrain of the Earth has changed to allow for dry land.
What mechanism was used for this? Apparently the mechanism is plate tectonics.
Here’s how one creationist web site explains it:
"The catastrophic plate tectonics model gives a mechanism for the deepening of the oceans and the rising of mountains at the end of the flood."
As the new ocean floors cooled, they would have become denser and sunk, allowing water to flow off the continents. Movement of the water off the continents and into the oceans would have weighed down the ocean floor and lightened the continents, resulting in the further sinking of the ocean floor, as well as upward movement of the continents. The deepening of the ocean basins and the rising of the continents would have resulted in more water running off the land.
The collision of the tectonic plates would have pushed up mountain ranges also, especially toward the end of the flood.
Of course none of this can be demonstrated in any way.
Moreover that explanation leaves unanswered questions elsewhere. For example, it speaks of the “ocean floors” cooling. That is in reference to the hot springs that supposedly were the “fountains of the deep”. The water coming from such springs is at a temperature of 400 degrees centigrade. If there was a significant enough number of such springs to allow the Ocean floor to collapse, there would be significant enough heat produced to boil all of the inhabitants of the Ark.
In regard to plate tectonics, most scientists believe that they are a very real phenomenon. However they work very, very slowly.
A Global Positioning Sensor (GPS) has been put on top of Mt. Everest during one expedition to the summit. That sensor shows that the summit is rising, presumably due to of plate tectonics, at the rate of about 3-5 millimeters per year. That is typical.
Since Mt. Everest is now nearly 30,000 feet above sea level, more than five miles of combined movements (movement down by the sea floor and upward by the mountain tops) must have taken place. At the maximum rate of 5 millimeters per year, that much elevation change would have required something like 2.5 million years.
But the flood only took one year.
Furthermore, this must have taken place in a very short period of time. Movement of the tectonic plates is associated with earthquakes. Yet written records (considered to be the most reliable source of information by most creationists) don’t mention massive earthquakes as would be needed for such massive tectonic plate movements. In addition to unimaginably huge earthquakes, large tsunamis would have been constantly occurring as the ocean floor fell. Such tsunamis would have devastated a large wooden boat as the Ark. There are no records of such massive and constant geological events.
In summary – there is no rational solution for where the water went after the flood.
 http://www.christiananswers.net/q-aig/aig-floodwater.html, referenced on April 24, 2008
 http://www.thetech.org/exhibits/online/everest/about/platetectonics.htm, referenced on April 24, 2008