Wattamolla is a sheltered cove in the Royal National Park south of Sydney. It has a large saltwater lagoon behind an ocean beach. Swimming here is absolutely wonderful. You can have a bodysurf in the cool ocean, then swim up the lagoon to the falls tumbling off the cliffs (avoiding the thrillseekers jumping off the cliffs). You can swim further up the north arm of the lagoon to Wattamolla Creek and follow the (comparatively) cold freshwater creek upstream to a series of waterfalls and an ephemeral swimming hole. I love sitting on the rocks under the cold force of the falls, then walking back down the creek to wallow in the warm lagoon.
Wikimedia Commons: Author: Klaus-Dieter Liss (Kdliss)
I have been coming down here to swim and to bushwalk for almost 25 years. I could never grow tired of it.