It’s 7:30 p.m., the sun has been down for about an hour, and I’m getting ready to plunge into the inky black water of the Great Barrier Reef. Below me is a group of hungry whitetip reef sharks, hunting for snapper and baitfish, circling our boat. I step off the Spirit of Freedom, a 122-foot liveaboard home to 26 overnight guests, floating near the central ribbon reefs off the northeast coast of Australia.