What is the Biblical Basis of a "water canopy", and is there scientific evidence to support it? Some Creationists have asserted that in the pre-flood world that there may have been some sort of water vapor canopy that perhaps doubled the air pressure and caused things to be much healthier than they are today (post-flood).

What is the biblical basis for this and what is the scientific evidence the proponents use to support this theory?
In Genesis 7:11, we read:


  11 In the six hundredth year of Noah’s life, on the seventeenth day of the second month—on that day all the springs of the great deep burst forth, and the floodgates of the heavens were opened. 


The implication here is that water came from two directions - springs from the deep (i.e. underground water) and from above (the floodgates of heaven).  Extending the metaphor ever so slightly, a floodgate is holding something back - according to the Institute for Creation Research, that is the canopy.  

The Institute for Creation Research has published this paper to explain the physics behind it:

This study, however explains some of the problems. Amongst these problems:


A water canopy of more than about 40 feet would, by the pressure, boil itself away
A water canopy of more than a few inches would also block all the sunlight.


In the link, however, the paper argues that these problems are not insurmountable depending on the assumptions made about the canopy, but I'll admit, it gets a little stretched for my tastes.