How deep could someone dive breathing air through an unblockable tube to the surface?

When I was a kid, around ten, I tried that with a garden hose in a pool. I thought it would be really cool to sit on the bottom and look up. My plan was then, after doing it in the pool, to then try it at the local marina, where I would then, I thought, earn money by scavenging for things under boats or renting my services out to boat owners.

I had high hopes.

At the same time I was being cautiously thorough, as my first idea was to jump off the pier, hose in hand, with bricks in my pocket. What could possibly go wrong I thought. And thinking again, I thought, I might lose my mask, I might let go of the hose…

So in the pool I went, walking from the shallow end to the deep end.

Was I surprised I could barely submerge my head!!!!

So, yes, a few tens of centimeters at the most.

Now I have grown up and know why. Has to do with our squishy lungs and the quite high pressures we are subject to at even shallow depths.

To illustrate that to novice divers, I take a water bottle, jump in a pool, at the bottom I blow air into it and close the cap. I surface and pass the bottle around before opening it. It is quite impressive, the bottle is hard from internal pressure and when opened, water spews out faster than a shaken bottle of champagne.