The way to discover - and explore - Sydney's underwater world is to go snorkeling, scuba diving or deep-sea diving.
Diving in Sydney
But yes, you can go diving in Sydney, particularly in the outer harbour areas and along the rocky sections facing the Pacific.
Underwater, there are wrecks and interesting rock formations to explore.
Among sea creatures you may meet are fish like the Port Jackson shark, wobbegong, snapper, bream, silver batfish, blue groper, and weedy sea dragon, as well as giant cuttlefish and giant ray.
Lessons, diving packages
There are any number of diving trips and packages offered in Sydney, and these can include diving lessons which give you the standing of an accredited diver.
If you're in The Rocks area, the Sydney Visitor Centre there is but one of your sources of information.
On the Net, particularly if you'd like to organise your diving trip before arriving in Sydney, check out Diving in Sydney for some sites you may want to look at.