The Great Barrier Reef was one of the few things I knew about Australia before I decided to go there and started planning my trip. Of course I had to go and see it, although I am not a water person and I don't like swimming in the ocean. So when I stayed in Cairns, I chose a tour operator offering trips including a glass bottom boat, and I liked it. I am sure it is nothing like snorkeling, but still I enjoyed what I saw and was fascinated by the underwater world.
Later, when visiting the Whitsunday Islands, I did another tour and this one was even better because the water was so clear and still that I could see the corals straight from the boat, without having to use the glass bottom.
I think I don't have to write anything else - it is famous enough already, and a "must see" for most people when visiting Australia!