How? Surely you would find out almost immediately that you can't inhale against the water pressure and then just surface. It's not like you are getting to 15m before you struggle, IIRC for most people you will be struggling or completely unable to inhale by just 1m. Even with training you are at best adding a few cm.

but... how? I mean, don't you feel it before dying that you can't breathe? Or he had too heavy gears to swim back at the surface?

I had that same physics lesson. Young me was so excited bringing the end of the hose, covered by my hand, to the bottom of the pool. I was almost out of breath when I got down there because swimming down while covering the hose was hard, but I figured it wasn't a problem because I was about to be able to breathe through it. I was pretty panicked when I quickly jammed the hose end into my mouth and it tried so suck the air out of me harder than I could suck it in.

Are you thinking of regular length snorkels with a valve? Never heard of longer ones as it's impossible to inhale against water pressure as you dive, plus it would be difficult to hold onto a very long tube.

Would like to do some snorkeling but the sea around where I live has pretty limited visibility, at best you can make out where your feet are. Got a swimming mask though and have seen some tiny fish before. But it's also a steeply sloping gravel beach so you almost immediately would be beyond snorkel depth and have to be free diving to get to the bottom

Yes, snorkels exist. But the depth they are usable is limited to only about a meter, before you cannot take in air

what if you breathe in from the snorkel and breathe out into the water?

That too.

Achtuhally you're standing on the bottom of the ocean as soon as you take a step in the water