Lake St Clair, the southern end of the Overland Track and Australia's deepest lake, is the exception to the usual rule that Tasmanian lakes are too tannin-stained to dive. The Marine Life Network guide rates Cynthia Bay 4.5 out of 10 as a welcome change from ocean dives and an alternative when the sea is too rough: light penetrates to about 10 m, below which the water becomes dark and lifeless, so the best diving is in the 5 m shallows among small weed growths.

The guide describes freshwater crayfish (lobster-like, 10-15 cm, with large but harmless pincers), small shrimps and minute underwater spiders, with trout staying out of sight and fishing lures snagged in submerged branches. It notes that animals must not be removed (this is a national park) and that the water is much colder than the ocean, especially with spring snowmelt.

Around the lake there is world-class bushwalking, a visitor centre, cafe and accommodation at Cynthia Bay. It suits divers looking for something different, and snorkellers in summer, and it can be combined with a day walk to Shadow Lake or along the lakeshore towards Echo Point. The nearest hyperbaric chamber is the Royal Hobart Hospital, two hours away, and diving at altitude (737 m) requires altitude-adjusted decompression planning.

The lake lies within the Tasmanian Wilderness World Heritage Area, and visitors should follow Parks and Wildlife Service guidance, including checking and cleaning gear to avoid spreading freshwater pests and diseases such as chytrid fungus and didymo between waterways. Rangers at the Cynthia Bay visitor centre can advise on any current restrictions on in-water activities.

Cynthia Bay is at the southern end of Lake St Clair, 5 km from Derwent Bridge on the Lyell Highway, about 2 hours (175 km) from Hobart. Park at the visitor centre (Parks Pass required) and enter from the shore nearby; the carry is short. The Lyell Highway is winding and can have snow or ice in winter, so allow extra time and check road conditions.

Lake St Clair is at 737 m altitude in the Central Highlands; strong westerly winds whip up waves on the lake, and snow is possible in any month. Water is coldest in spring with snowmelt. Visibility is good for a Tasmanian lake (light to about 10 m) but deteriorates after heavy rain.

Calm summer mornings are best.

Stay in the 5 m shallows, where small weed growths and freshwater crayfish are found, and avoid the dark, lifeless deeper water. Return before getting chilled. Turn at a third of gas to allow for the swim back and a possibly harder exit, finish with a 3-minute safety stop at about 5 m if the depth allows, and exit where you entered unless an alternative exit has been checked beforehand.

Altitude diving at 737 m: set computer for altitude and allow time to equilibrate after driving up from the coast.

No dive operator. Occasionally dived by Hobart and Launceston club members and independent divers as a freshwater novelty or bad-weather alternative; bring all gear and filled cylinders.