Common Stingaree
Common Stingarees, as their name suggests, are everywhere in Sydney. They are perfectly happy in murky water, so they can even be found right up the estuaries.
And yes, they do sting - they have a venomous barb that can snap off in your foot if you actually manage to step on one.
Fortunately Stingarees are not too fond of being stepped on and they're off in a flash long before your foot even gets close. You'd generally have to glue one down before treading on it to get stung (and that's definitely not easy underwater).