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You wrote:
>.... The pressure MUST be equal between water and air at whatever 
>depth you are at due to basic laws of physics ( as stated previously).  
>The air is going to be compressed until it exerts enough outward force 
>for there to be equilibrium.  So the whole idea of pressure here is 
>irrelevent....  

The pressure in a submerged bladder containing a compressible gas such as
air is not everywhere equal to external pressure as you suggest. This is a
common misconception. The pressure in the bladder will be equal to the
ambient pressure at the deepest part of the bladder minus the pressure
exerted by the weight of the column of air from the point in question to
the deepest submerged part (this assumes the bladder is not pressurized and
is allowed to freely expand and contract). Since the weight, actually the
mass densities of air and water are significantly different one can
generally neglect the pressure exerted by the weight of the column of air
and make the assumption the pressure in the bladder is the same as the
ambient pressure at the deepest submerged part of the bladder. This
application of physics is essentially an extension of what you propose and
is due to the relative density differences between air and water. The
compressibility of the gas essentially determines the displaced volumn. So
in short the pressure at the top of the bladder (assuming a vertical
orientation) is greater inside than the ambient outside pressure.

A real world application includes ambient pressure underwater habitats
where the entrance/exit trunk is located at the bottom of the habitat.
Commom literature will have you believe the pressure is equal everywhere
across the wall of the habitat and thus significant structure (other than
to resist buoyancy forces - which can be significant) is not required.
However this is not so. If you were to have a habitat that was 10ft tall
for example the pressure difference at the top of the habitat would be
about 4.5 psi which when distributed on lets say a 4'x8' sheet of plywood
would apply about 10 tons of force. Bringing this to bear on BCs, a 1.5ft
height bladder may have a differential pressure of .67 psid, which
relatively speaking is significant. 

No flame here, just a little more information on the subject,
Doug Chapman



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