Just qualifying a bit of what stomps has said, clearest water will alweays be found about islands, the further from the mainland the better to avoid run off pollution.
Obviously with islands, there is a transport/cost issue and though box jelly fish are a problem up north between say November to May, they are more a mainland problem rather than for islands.
Of the islands, North Stradbroke on southern end of Moreton Bay (mouth of Brisbane river) is probably about the cheapest to get to/stay and as stomps says has superb beaches, beautiful water but not much snorkelling that I know of - and there are deep water channels off popular northern beaches which sharks frequent.
There is great sand in many locations about Australia from WA, the southwest/southern area in particular and also many more isolated stretches of beach heading to NW of state.
From South Australia right around Victoria and up the NSW and Qld coast you have great beaches, water clarity varying with closeness to populated areas and the further south you are the colder the water is.
The Gold Coast has good sand but it is all mainly a surf coast, little snorkelling opportunity and quite dangerous in many places.
Sunshine coast is similar though with less development.
Fraser Island has great beach stretches but a NO No for swimming/snorkelling owing to rough water and known to be an area where a lot of sharks are, but lake beaches are great.
If you are heading as far as Cairns, Great Keppel Island near Rockhamptom,
Hook Island (a great ideal/cost compromise to stay) and Whitehaven Beach (on a day trip or several days sailing trip -
Definitely not to be missed), both in Whitsundays, Magnetic Island (Townsville) and Dunk Island (Mission Beach) and Fitzroy Island (a day trip from Cairns) others worth considering.
Typically, a day trip to a barrier reef island/atoll with lagoon for by far the best snorkelling will set you back anywhere between $100-200 depending on what you choose.
Northern Queensland islands have warmer water, some off beach snorkelling and you may sight harmless reef sharks rather than larger bitey types.
I'm even longer but not sorry
