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Where do you think that pressure comes from? It is caused by the amount of air above the ground. A one-inch square column of air from the surface to the sky weighs 14.7 pounds.....thus 14.7 pounds per square inch pressure.
you are on the right track, but density alt corrects for temperature and humidity. Calm air has nothing to do with this. But it is exactly the density of the surface air that matters to divers.
I think it does matter, but it’s a hard concept to understand and goes above the math ability needed from the various agencies. .
A typical summer day in key largo is say 90 degrees F and 70% humidity. That gives a pressure altitude of 2400 feet. So standing there on the dive boat is the same ambient pressure as being at 2400 feet elevation. .
The density will affect if you sink (air), float (dead sea), or somewhere in the middle. An airplane cares about the density so it can float on a cushion of air. If it can't float, it can't fly. You need dense enough air for a plane to fly.
Another way to think about it is that your aircraft things -- density and pressure altitudes -- are point quantities, which is what the airplane cares about. The atmospheric/barometric pressure is an integrated quantity...all the atmosphere above the diver.