I won't care to mention the big ones such as the Great Barrier Reef, but I'll add a couple smaller unspoiled ones in my area.
My favorite has to be Little Salmon Bay on Rottnest Island outside of Perth, WA
. I have been there a couple times and have never seen anyone else there, almost no one has heard of it, and it isn't one of the acclaimed snorkeling spots on the map. But, every time I have been there, there has been lots of fish of different sizes and colors, and I've seen stingray's every time. One time we even saw an octopus. The next one has to be Little Parakeet Bay on Rottnest as well. On one of the sides of the bay, there is a very rocky bottom with lots of fish hiding. In the middle there is a sudden deep well, almost like a small crater, with lots of fish (although it seems perfect for a shark hiding spot!). The other great thing about Little Parakeet bay, is that when you are not snorkeling it is better than Little Salmon bay, there is less wind, more beach to get a tan, and better for swimming and bringing a boat in. It is never packed, but there is always other people there.