Just returned on Celebrity Solstice 14 night cruise to Australia & New Zealand

Google "wave dancer low isles".

Of all the GBR tours I've done, this was the nicest (And THAT trip I took four times already)

The Sailaway trip to the Low Isles is pretty good too. The snorkelling is actually quite decent there.

Check out Reef Magic. I have not been as I am planning a trip, but they get good reviews on TripAdvisor. They take you out to a platform where you can snorkel, go in a glass bottom boat, or a semi-submersible, or they even have an underwater viewing area. You can also scuba or SNUBA or helmet dive. I also heard that the coral on Green Island is pretty washed out.