Shelly Beach

http://members.ozemail.com.au/~scse/shbeach.html
Visibility is generally very poor. This site makes for a good night dive and
there is some junk (a motorbike and an old oven) in the bay in approx. 5 m
of water.  A maximum depth of 14m can be reached if you venture out of the
bay. Inside you only will get about 5-6m.  On rare occasions you may
encounter the vicious "brown spotted diaper fish", a rare species, which can
be found hiding in amongst the kelp in the shallows, usually on a hot
summers day after plenty of toddlers have been frollicking on the beach.

http://www.asitis.com.au/~adlind/ddescrip.htm#Shellyb
A very easy and entertaining dive in a sheltered part of Twofold Bay. Just
wade in and swim to the outside edge of the exposed rocks and down you go.
There are two main dive options here. Firstly you can follow the natural
reef to the right and link up with the long Snug Cove breakwater .
Navigating is easy from here as you follow the sandline out to 10 -12 m. The
second option is to go left and explore the natural reef which is covered in
weed, sponges and gorgonia fans. There is a layer of silt covering the sand
which can easily blot out much of the available visibility if divers are
careless (student divers learn minimal impact techniques quickly if they
want to see anything). Skill Level: Beginner

http://www.divesydney.com/divesitesb.htm
The only beach on the entire east coast of Australia to face west, Shelly
Beach occupies a scenic comer of Manly's ocean beachfront. White sand and a
natural reef of tumbled boulders and rocks descending to a maximum depth of
14 metres,  For beginners and experienced divers alike an escorted, Dive
Centre Manly, Shelly Beach Discovery Dive is an experience that will long be
remembered.