It is difficult to conceive but necessary for divers to understand because partial pressure affects the amount of nitrogen and oxygen dissolved in the bloodstream of divers breathing air or mixed gasses under pressure. Too much oxygen can cause oxygen toxicity and too much nitrogen released too quickly can cause bubbles to form in the bloodstream resulting in the bends.

Breath holding and diving to the bottom of the pool is different because as you dive down the air compresses in your lungs and as you come back the air expands back to the volume it originally was.

If you go to the bottom of the pool with scuba on you are breathing compressed air. You would not be able to breath surface air through a tube because at say 11 feet there would be almost 5 psi surrounding and compressing your lungs. So on scuba you are actually breathing 5 psi air at 11 foot depth. At 100 feet you are breathing almost 45 psi air pressure. But the water pressure surrounding the lungs is also at 45 psi so breathing is as if you were at the surface. The pressure is equalized on both sides of the lungs. If you were to hold your breath to the surface from the bottom of an 11 foot deep pool breathing from scuba your lungs would be holding 5 psi in them at the surface which is impossible. Instead your lungs rupture and foamy blood froths from your mouth and you are dead momentarily. That is why you never hold your breath when using scuba especially while ascending.

33 feet sea water depth is the same pressure exerted as the weight of an atmosphere of air. From sea level to outer space. That is the difference in the density and thus the weight of water compared to air.

John