Handheld Tanks

If you want to stay underwater longer without getting scuba training then look into free diving. At shallow depths on scuba your biggest issue is a lung embolism, you are at the most risk at shallow depths, the training will keep you alive.

Dcs (the nitrogen in your blood you are referring to) is the least of your problems.

And the fact that this is the only risk you see proves my point. You are much more likely to be dealing with lung overexpansion syndrome, alveolar ruptures a resulting lung embolism and other issues.

Getting owd certified costs about as much as one of those toys you're intending on betting your life on. An apnoe course is even less.

This is not a DIY project.

Edit: ah. Scuseme. I read the first "not" in your response as "no", which gave it quite a different tone. Sorry bout that. But my point stands. If you want to go cheap: apnoe. 5 minutes of breath hold diving can be trained surprisingly quickly. But. This equally demands qualified, professional training. These things can turn south reeeeally quick.

Is the only next step training and certification

Yes. You should just go get certified. Taking a shortcut here is not a good idea.

I'll put this in a different comment to avoid writing a novel.

So. Without giving you any ideas.

The problem you're dealing with is the density of water and the pressure it exerts on your body.

The deeper you go the higher the surrounding pressure becomes. Your lungs need to counteract that. That's why divers use compressed gasses. And that's also, why you can't use an extra long snorkel.

(To give you a fun visual: Would you attempt to breathe through a long enough hose leading to the surface at depth, your pulmonary organs would get squeezed like blood oranges and likely actually end up being squeezed into said hose... there was a fun mythbusters episode on a similar scenario once.)

So you need to counteract that. This is in practice either done by carrying pressurised gasses in a tank or supplying in it through a compressor that's usually operated from the surface. The latter is either left to professionals (sat divers, or the infamous mkV suits) or to clueless untrained tourists under the names of snuba or hookah diving - which are lawsuits just waiting to happen.

But that's just the very basic stuff. Diving combines a lit of aspects of physics and medicine. And while it can be done perfectly safely, it should definitely not be tinkered with as long as you don't understand what you're doing. Many others have paid these prices already, so we don't have to.

Edit (again): while apnoe doesn't bear (most of) those risks, it comes with its on challenges - and its own unique ways to die. Again. Don't DIY. Get training. Freedivers pass out. Quite regularly. That's a bad thing. Especially when your face is under water.

A floating buoy and a hose is probably safer than putting compressed air in your body at depth.

I honestly don’t know if it’s possible to breathe low pressure air in a high pressure environment, but I’m assuming if that fails you’re just freediving and can come back up.