It varies.

There are various kinds of fish, coral, etc. at various depths, depending on
where in the world you are diving.  You aren't likely to see any coral, or
many fish for that matter, in an inland lake.

Like I said, it varies and completely depends on local and sometimes even
specific dive sites at those locals.

Get certified and then go diving and find out.  After all, you have a
curious nature.  :^)

Other than photos of stuff that is only found deep I'd guess that most of the photos
you see are from fairly shallow depths. There's more ambient light closer to the
surface, so all else being equal there's a better chance of the shallow pictures
having nicer lighting. Your air (and no-deco time, naturally) last longer when you're
shallow, so many people spend much of the dive there.

I'd guess that most stuff can definitely be found at depths of less than 30 feet.
Many fish that can live at depths of well over 100' may also be found at depths of 10
to 30'. Even if 90% of species can't be found above 100' I'd guess that close to 90%
of the biomass can be found above 100'.

I'd guess that the vast majority of the stuff you see in magazines *can* be found
above 40'. Buy or borrow a book(s) on reef fish, and reef critters that include info
on the depth at which species are found. Here's a good example:
http://www.amazon.com/exec/obidos/tg/detail/-/1878348078/002-5194665-1264841?v=glance

Life in the oceans is scattered from less than a foot, to much more than 100
feet, but there's enough fish, coral and other life in the first 30 feet of
ocean to last the casual spectator a lifetime, particularly if you're
willing to travel.

Lee

Generally speaking, the photo's in recreational dive magazines are taken
at what is known as "recreational" depths, which technically means  
130fsw max, and generally probably less than 100fsw...most of them were
probably taken at around 50fsw.

Once you have some experience diving, you can take a look at a photo and
refine your guess as to how deep it was taken.

For example,

(shuffle, shuffle):  

Cover photo on Aug 2004 issue of Sport Diver:

Image is of a snorkeler above a nurse shark, and he's clearly on the
surface...therefore, shot was taken at <10fsw.

Cover of July 04 Sport Diver:

Close-up of what looks like the face of a Great White Shark.  Dark
background, which would generally infer more water between the
photographer and the surface...but there's some cues in the background
that also infer that the surface appears to be fairly close.  I'd guess
this was shot at maybe 25ft, plus or minus.

Cover of June 04 Rodale's Scuba Diving:

Image is of a shark overhead blocking sun, background shows a shark feed
underway, with kneeling divers wearing single tanks (feeder has
doubles).  Note in corner credits the shot to the Bahamas.  We know from
other sources that these feeds are typically done in the 30-50fsw range.  
We can then use this shot to help estimate others.

Generally speaking, most of the life is where most of the sunlight is,
which means it will be shallow.  However, the surf zone is too rough for
a lot of critters, so there is a tendency for the critter life that
we're interested in looking at "starting" a bit deeper.  

I think the answer to your question is that there's "plenty" of life in
the 20-50fsw range, as well as stuff shallower and deeper than this too.

Generally, what you'll find is that the critters each have their own
preferred depths, some shallow, some deep, some in between.  For
example, if you've seen photo's of blood red sponges in the Caribbean,
this is probably the Strawberry Sponge, and it is rare to find it
shallower than 40fsw.  

This is probably because it is red only as a manner of convenience:  
when you eyeball it in the wild, it looks nearly black because water
absorbs the red spectrum part of light first, which means that red
objects appear black at roughly 40fsw or deeper.  The reason that the
sponge chose to be red instead of black is supposedly because red is
biologically "easier" than black, so it gets the benefit with less work.

And its a favorite UW photo subject because the camera's strobe restores
the full light spectrum so that we can see its true color.

-hh