Vapor canopy. This model, proposed by Whitcomb & Morris and others, proposes that much of the Flood water was suspended overhead until the 40 days of rain which caused the Flood. The following objections are covered in more detail by Brown.
How was the water suspended, and what caused it to fall all at once when it did?
If a canopy holding the equivalent to more than 40 feet of water were part of the atmosphere, it would raise the atmospheric pressure accordingly, raising oxygen and nitrogen levels to toxic levels.
If the canopy began as vapor, any water from it would be superheated. This scenario essentially starts with most of the Flood waters boiled off. Noah and company would be poached. If the water began as ice in orbit, the gravitational potential energy would likewise raise the temperature past boiling.
A canopy of any significant thickness would have blocked a great deal of light, lowering the temperature of the earth greatly before the Flood.
Any water above the ozone layer would not be shielded from ultraviolet light, and the light would break apart the water molecules.