Not sure what Darling Harbour has to do with snorkelling...you wouldn't want to get in the water there for fear of being hit by a boat. Plus, I'm sure it would not be allowed to snorkel in the middle of the city harbour!Manly is definitely good, but if you can make it out of Sydney, I do recommend Jervis Bay.It is about 3 hours south of Sydney and boasts some of the best diving spots on the south east coast. I was down there recently and spotted a large pod of dolphins...it was really cool to see them so close..

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