In William Bay National Park near Denmark on the south coast, Greens Pool is a wide, calm lagoon of clear emerald water sheltered from the Southern Ocean swell by a wall of massive granite boulders. The water is still and shallow enough for children, warmer than the open sea, and clear enough to snorkel among the fish. A short walk over the rocks leads to Elephant Rocks, a cove of huge grey boulders like a herd wading into the sea. Backed by peppermint forest and white sand, it is one of the most beautiful and family-friendly swimming spots in Western Australia.