How deep underwater could you breathe using a tube which breaks the surface until the water pressure makes it impossible to inhale air?
First, a sanity check.  Snorkeling isn't generally done at much depth, and this question reflects some subjective sense of that.
My guess (after much confusion) is the table they're providing is apparently the pressures that the subjects were able to apply, when striving to breathe in or out at various levels of &quot;Lung Volume&quot;.  But what is &quot;Lung Volume&quot;?  Not Total Lung Capacity - is it Tidal Volume, Expiratory Reserve Capacity, something else?
If I read it that the person, at 22-54% &quot;Lung Volume&quot;, can apply only 53 cm H2O of pressure differential while inhaling, that implies that at this depth of external pressure, they can't inhale.  (Oddly, if they lose the battle and go to 1%, the data says their ability to inhale is reduced!) In theory the fluid pressure of water or air on the body should be similarly positioned.
Even so, there are two things that make me doubt this.  First, the inability to inhale should be coupled with a greater ability to exhale.  So I still wonder if sort of shallow breathing might still be possible at the limit of the expiratory reserve, or even in the range of &quot;residual volume&quot;, as the external pressure is opposed by connective tissue.  Second, the diaphragm and the external intercostal muscles are, well, muscles, so it should be possible that someone could train those muscles well and resist more pressure.