H2Andy:so in a normally occurring air pocket, if you surface into it, your gauge would read zero, right?
Not if the water level in the pocket is below sea level.
Think about it: you ascend to the water/air interface. You slowly raise your arm into the air pocket. Do you think the pressure gauge will instantly drop to zero? It's a good thing that's not how it works or you'd be bent worse than a pretzel in there.