A menacing fin pierced the surface and sliced toward us. A great blue shark—three meters in length—homed in on the scent of blood like a torpedo. As my wife, Melanie, and I watched several large sharks circle our seven-meter Boston Whaler, a silver-blue snout suddenly thrust through a square cutout in the boat deck. “Look out!” Melanie shouted. We both recoiled instinctively, but we were in no real danger. The shark flashed a jagged smile of ivory saw teeth and then slipped back into the sea.