The first time it happens, it's, a, completely by accident, and b, completely for real.
You're looking for something in your bags (gravel -- or sand or dirt, but gravel by preference, because it is otherwise useless -- so you can build a walkway through the water as you go) and you forget that you're in water, forget that you have to keep swimming, and everything goes red; alarmed and disoriented, you try to find the surface, but you don't get there in time. The world goes reddish-grey, feeling sort of like a soft explosion, and then you wake, head reeling a bit, in the bed you slept in four nights back.
(You don't need a lot of sleep, but sometimes it can be honestly tiring, toiling away in the dark, and torches can help but not nearly enough. Still, at least you're waking up here, in the cobblestone fortress that you call home. Used to be, once upon a time, that when you died you'd wake up ... elsewhere, the place where your first memories are. Once upon a time, you tried to keep your home near that elsewhere, even if resources were scant, because otherwise you couldn't always find your way back home again.)
You stumble out the doors and find where your equipment lies scattered, and you pick it up, piece at a time, armor and weapons and chunks of ore and piles of wood and unlit torches for nighttime. Most of it requires diving underwater, because that's where you --
Well. Where you died; but it's not that simple, because it wasn't anything like the other times you died (you've never died to a zombie, or spiders, but skeletons have gotten you once or twice, and then there's the goddamn creepers with their utter silence until they ssssssssssBOOM and you're dead as soon as you know they're there; and you've died smothered in sand (once, frustratingly, smothered in a /tree/, and you'd flailed around with your axe to no avail) and died by falling off cliffs and falling into caves; and definitely you've died to lava more times than you can count). Those were annoying, especially when you couldn't recover the equipment for one reason or another.
But this? Okay, it was annoying, but it was also sort of ... well, /fun/.
You bring everything back to your home base and dump it all in chests for good measure. Every last piece, even your weapons and armor (made of diamond, battered but still in decent shape). You feel naked -- it's been a long time since you've gone outside without muffling yourself in layers of leather or metal, longer still since you've let yourself wander without at least a sword for defense -- but also free.
You walk to the ocean, step in waist-deep, and stand for a moment on the precipice of deeper waters before you step forward and let yourself float downwards.
When you're doing it on purpose like this, you're a bit skittish; the first few times you kick back up to the surface long before you were ever in any danger of running out of air. But you play with it, dare yourself to stay down for longer, to see how deep you can go and still get to the surface in time. Out of the water, breathing is so normal you never think of it (except for the breathing that others do, the slow ghastly moan of the zombies or the lack-of-breath of the creepers or the offended bleat of a cow when you punch it in the face). In the water, where you have no option for breathing, you feel an excitement you don't feel other times.
How long can you go?
You push it, even though you know your capabilities aren't going to change, and sometimes you breach the surface of the water with a splash and a gasp and a rush of air that dizzies you, and sometimes you find yourself holding back long enough that your lungs are searing agony by the time you make it, and sometimes you get the grey-red fwoomp of death while you are still under water; but every time is as awesome as the last, as thrilling, as captivating to your attentions.
Night falls, and you go back inside, because you're not /stupid/; and you put on the armor and the weapons and grab a handful of torches and wait for daylight.
It takes you two days (and part of the night inbetween, stumbling against dark shapes in the darkness, unable to tell level ground from crevasses, unable to tell sheep from spiders except by the eyes, unable to tell trees from creepers except for the fact that the former doesn't kill you, but then you fall halfway down a cliff and decide that digging a shelter to wait out the rest of the night is the better part of valor) to find what you're looking for. Snow on the ground, ice thick and solid where water once was.
There, you build yourself a temporary shelter made of dirt and torches, and you put your things in a chest so they'll be easy to get at, and then you play some more.
At the border of the ice, where falling snowflakes change to a heavier rainfall, you dive into the water and head underneath the solid cover. The water feels the same as elsewhere, still a cool embrace preventing you from breathing, and you wait until your vision starts to flash red before you start hacking at the ice above you.
Sometimes you get a hole made in time (and then you dive back under and keep your breath as you wait for the ice to creep back and close up the hole). Sometimes you don't.
It doesn't matter.
After a while (you have a short attention span, and a vague memory of half-explored caverns underground that you keep meaning to go back to) you start building a monster trap -- a chance to finally get back at the skeletons with their stupid arrows and the creepers with their stupid creeping and the zombies with their stupid need for brains -- but just for fun, you build it over water.
Partly, this is so that when you fall off during construction (which happens rather a lot), you won't die.
But partly, it's so that when you fall off during construction, you can float there for a bit, nurturing the gulp of air that you managed to get while falling, and see how long you can last.