I've done a good amount of diving: In Hawaii where the ocean is clearer than glass, so that you're twenty feet under and can see all the way up to the cathedral arch of cloud and mountain and you're never quite sure whether this thing you swim through is sea or sky, or if it even matters. In San Maartin where the sand is made from translucent black obsidian and annelids in their ivory shells unfold chrysanthemum tentacles scarlet as silk and drag at your shadow, a strange shimmering three-dimensional thing caught in the glass sands. In Bermuda where the coral has a jellyfish sting and touching it unwary leave puzzlebox lines on your skin for weeks.
My parents were divers, and while this is my first adult cert (and the murky Puget Sound, with its downed airplanes and sea stars two feet across and haloclines drawn in sunset-colored curtains across the deeps, and currents that wind like moebius-strips and eight-degree water is very different), I held a kids' cert until I was too old for it.
So I'm lucky to have first-timer gfish with me, to remind me of the sheer, "I'm breathing underwater! Wow!" wonder (and because, frankly, he's much less likely to absent-mindedly forget to turn his air on than I am, despite my experience. :) ). It's the sort of thing one should never take for granted, no matter how young one might have been when one first tasted regulator air.
This is a thing physics teaches us: that light bounces at a reflective angle off a smooth flat surface. That the world is made up of smooth flat surfaces, like painted cardboard, fitted together, and darkness beneath. Tables and books and chairs and the hand that rests in yours, cardboard mosaics.
When I was doing computer animation, I really loved modeling, trying to get all my computer-generated triangles lined up to make the illusion of a real thing, paint and shading and little bump-map hacks. It was like building scenery for a staged play, prop this up, shade that a bit to look more shadowy, making reality of cardboard and vice versa.
After a while, though, it became increasingly obvious that for all I knew, the world was actually like that. That the way I thought of the world, the way I interacted with the world, the flat dead edges of stone and sea and sky - it might as well have been stage scenary for a play.
And if you're thinking of the universe that way, as cardboard scenary, you're doing something wrong.
Learning how to Make Things helped a lot, taught me to look under the skins of things to see movement and touch, taught me to see glow and hammerscale and the midnight-purple wash of temper color in a steel doorframe. Taught me that a word printed on a page is a motion, a touch, is where ink and metal and paper dance for a moment. Taught me to see the warmth of planed wood, the way you can feel the notes of the saw against your palms, in the smooth varnished surface of my office door. To know the way solder uncurls and gathers itself on a circuitboard and the gull-swept honey of molten glass and the way metal falls in spiral curls from the lathe, made the things around me real.
Learning how to Read Things helps, too. One of my colleagues, a rainforest researcher, refers to the 'wall of green' - when people first go to her forests, all they see is green, far as the eye can see. But you can learn the language of it, learn how to read it, like learning ink splotches are letters and words. And it changes the way the universe works for you, makes forest a written thing, a breathing thing. Once you can see ink splotches as letters, it's very hard to see them as ink splotches again. Once you can read cedar and wild ginger and ivory-pale mushrooms, you can't just see the cardboard wall-of-green.
Ah, but diving! Diving teaches you that the sea is a real thing, that it is not a blue-grey polygon extending out to infinity, but a place you can learn and love, a country of shadow and song and shelter and the weight of light across your upturned hands. You learn the feel of currents, surge and tide and rip, the heartbeat of the sea, and the way they trace out the edges of stone and sand and ship, write the shapes of things on your skin.
I think it's generally a good idea to make sure you live in the most complex universe you can build for yourself. (Not just things - people too. Don't keep anyone around if you could replace them with a shell script.)
So diving has been much happifying, even the wierd underwater dreams.
(Fellow pilots will recognize a lot of this, given that this is the same sort of thing flying teaches you about the sky-as-a-place-you-can-learn. :) )
See also: gfish's post.