Take a regular snorkel, hang onto the side of the pool and, whilst breathing through the snorkel, hold your head underwater so it is completely submerged but the tip of the snorkel is still above the water.
If you can draw air down at all you're doing well, if you can draw air down in a normal breathing pattern for any length of time time then you are doing extremely well.
Yes, just bobbing vertical in the water breathing from your snorkel your lungs may be a foot or so below the surface and the effect on breathing resistance is very noticeable if you pay attention. You can feel this when vertical head up underwater with a scuba regulator as well. Pressure is balanced to mouth depth and you have to pull that next foot. If you go head down breathing will be very easy. When swimming prone in good trim the regulator and your lungs are at nearly the same depth and life is good.
Your ability to breathe is based on the movement of your diaphragm that causes a very minor change in differential pressure that drives inhalation.
So it won't work at 1m, let alone 30.