Cairns is one of the best places from which to explore Australia's Great Barrier Reef: the longest coral reef in the world (2000 km). While it's expensive, this day trip is a definite must!!!
Whether scuba diving or simply snorkelling, you'll see an incredible variety of marine life, coral and, with any luck, sharks! Don't worry; the sharks that frequent these warm waters are harmless to humans.
Find bellow the Smart Choice company that we recommend for taking a catamaran to the Coral Reef (you can dive and snorkel on the same trip). Though if diving is not your thing, then we recommend you to do a day trip to the Green Island. An amazing white sand cay in the middle of the reef. There you'll be able to do some snorquel and relax on its beautiful beaches.
If you have a scuba diver's certificate, in addition to diving, we recommend doing some snorkelling as well, for no other reason than because it's simply amazing. If you're not a certified diver, don't worry: there are tons of things to enjoy. If you've always had an itch to try scuba diving but were reluctant to get certified in seemingly "less serious" country, now might be a good time to scratch it. Australians in general, and specifically the company we recommend, are 100% serious and safe in these matters. For a surcharge you have the possibility of taking an introductory scuba diving course.