Hi Shireen,
Well, my methods of picking dredging sites are not very sophisticated
- it pretty much depends on where I can find boat-launching
facilities, in an area where the bottom is not too rocky.  As you
said, coral reef bottoms would not be suitable for dredging - but
here in the northeastern United States we don't have to worry about
that.  There is only one species of coral here, and it grows in
little flat patches on rocks - no branches or brambles, and no reefs.
 The rocks themselves are the main problem to be avoided.  Here in New
England (for those who might not know, New England is a collective
term for six states located at the northeast corner of the USA) there
are many rocky shorelines.  In some cases, the rocks are fairly small
(egg size to fist size) and smooth.  But in other cases the beach is
covered with rough boulders ranging from basketball size to truck
size.  Usually (though not always) the nature of the substrate just
offshore resembles that of the intertidal area - rocky bottoms off
rocky beaches, sandy bottoms off sandy beaches, muddy boittoms off
muddy beaches.  Even off rocky beaches though, you can usually find
sandy or muddy bottom if you get far enough offshore.  A good quality
depth finder or "fish finder" is useful in assessing the bottom type,
as well as the depth.  I don't have one however.  I get my depth
information the same way Columbus did - with a weighted "sounding
line".  Specifically I use a fishing reel with braided nylon line
that I have marked at intervals with various colors of fluorescent
paint.  A 16 ounce (450 gram) lead weight takes it straight to the
bottom, and I can read the depth by the colored marks where the line
enters the water.
Trying to dredge on a rocky bottom is not usually worthwhile.  The
dredge bounces and bangs along, often gets damaged, gets hung up on
large rocks, and may become irretrievable either by filling with
rocks, or more likely by getting wedged between two large boulders.
Actually it is the chain which leads the dredge that is most likely
to get wedged somewhere.  There was one fellow I corresponded with
years ago who did have some success dredging brachiopods
("lampshells") off rocky bottoms in the northwest.  Brachiopods look
superficially like bivalve mollusks, but they live very differently.
They attach to rocks by means of a stalk, and the shell sits atop the
stalk like a golf ball on a tee.  Apparently they were quite abundant
in his locale, and the dredge would clip off enough of them as it
bounced over the rocks to make the effort worthwhile.  However, he
also told me that he carried three dredges on the boat, and at least
once he lost all three of them on one trip.  I only lost one dredge
ever, while dredging on a flat mud bottom, 80 feet deep.  I'm not
sure what I "caught" (or rather what caught me) but I suspect it may
have been a sunken fishing boat known to be in the area.
Dredging on grass beds can be somewhat productive.  A small dredge
like I described in my previous posting won't rip up grass by the
roots like a large commercial dredge or trawl would.  On the
contrary, it just bends the grass down as it moves along.  This
prevents the dredge from digging into the bottom, and actually even
from touching the bottom.  Therefore you won't pick up many burrowing
bivalves; and, you won't get many gastropods that crawl on the surface
of the sediment.  What you will get is small snails that live on the
actual blades of grass (in some areas there are amazing numbers of
these), and also bivalves that swim, and come to rest on the grass,
or attach to it by a byssus (in this area, primarily pectens and
small mussels).
Dredging isn't just for deep water.  You can snorkle over a sand or
mud bottom in a few feet of water and not see a thing.  Yet a small
dredge towed along the same path may yield many interesting specimens
that were hidden just below the surface of the sediment.  In such
shallow water you don't necessarily even need a boat - you can tow
the dredge manually if necessary.  Make a large loop and put it
around your waist - or tie the tow rope to the middle of a sturdy
pole, and two or more people can work together, holding the pole
horizontally and pushing on it like a yoke of oxen.  Two variations
on this theme are - a hand dredge (sort of a mesh scoop attached to a
pole, that you can use manually, with a raking action); or screening -
shoveling sediment onto a flat screen of suitable mesh, and sifting it
through by shaking the screen (easier if the screen is held in the
water while sifting).  With either method, as in dredging, you don't
have to dig in deep - most specimens of interest are in the top inch
or two of sediment.
What are you likely to get besides shells?  Here's a partial list
from my experience - sand dollars, seaurchins, sea cucumbers,
sandworms, lugworms, tube worms, bloodworms, scaleworms, starfishes,
brittle stars, sea robins, toadfish, pipefish, sculpins, flounders,
cunners, skates, lobsters, spider crabs, hermit crabs, green crabs,
rock crabs, blue crabs, horseshoe crabs, sea spiders, barnacles,
amphipods, mantis shrimp, grass shrimp, coral, sea anemones,
jellyfish, comb jellies, sea squirts, sea porks, sponges, seaweeds,
dead shells, rocks, bricks, broken glass, bottles, cans, plastic
bags, wood, a grapefruit, a Batman action figure, a pair of pliers, a
Boston Red Sox hat - I guess you get the idea.  That's why I recommend
rubber gloves while picking through the catch.  Note: before you toss
back the wood, check for Piddocks and shipworms.  Before discarding
dead shells and bottles, check them inside and out for limpets of
various kinds, and chitons.
Regards,
Paul M.
Providence, Rhode Island, USA