Darwin is hot all year round, but the best time to visit for a swim is during the dry season, which runs roughly from April to October. Most natural swimming spots in and around Darwin are closed during the wet season because of the risk of crocodile attack. The Northern Territory Government maintains several croc-free swimming holes during the dry season, which are clearly signposted as such. Unless you want to be lunch for a giant reptile, don't swim anywhere that isn't signposted as safe for swimming.
Berry Springs - This series of two natural water holes is only about half an hour from the city centre along the Stuart Highway. The large, deep pools are fed from a naturally warm spring that gushes into the site at one end. The water is crystal clear and the pools are surrounded by thick jungle. It's very popular with both locals and tour groups, so try and get there early to avoid the crowds.
Litchfield National Park - There are several excellent swimming holes in this national park, located about 90 minutes south of Darwin along the Stuart Highway near Batchelor. The pick of the bunch is Wangi Falls with its towering falls and massive swimming pool. You can also try Florence Falls with its twin waterfalls or Buley Rockhole with its series of rock platforms and cascades. With this much choice, it makes for a great day out in the water.
Darwin Waterfront - The waterfront precinct is open all year round and the only crocs you will see here are on the feet of the tourists flocking to escape the heat of the Darwin city centre. The main feature is the wave pool, which has 10 different wave sets that change every twenty minutes, but there is also a saltwater lagoon for a more leisurely swim. The waterfront is located on the south eastern edge of the CBD on Kitchener Drive. A small entry fee applies.