India, May 23 -- Standing atop a mountain, looking out over Port Davey-an inlet of the Southern Ocean in southwestern Tasmania-Pete Marmion told me the quirky way in which he measures bodies of water. "It goes millilitres, litres, Olympic pools, and then Sydney Harbours," joked Marmion, a local guide with more than three decades of experience showing visitors around Tasmania. "Port Davey is three times the size of Sydney Harbour."

Sydney Harbour is 12 miles long and has a total area of 21 square miles; it's one of the largest natural harbours in the world. From the top of the mountain, watching a torrential rain pour into the water far below, I thought Marmion's estimate seemed sound.

Port Davey's dark, tannin-stained waters lie at th...