Diving in South Australia: Sea Dragons, Jetties and a Sunken Destroyer

Picture standing under the collapsed tee of an old jetty, looking up at steel columns furred with sponges and ascidians, while a leafy sea dragon hangs motionless in the weed beside you. That is Rapid Bay, and it is the single image most divers carry home from South Australia.

This is temperate diving. Kelp, limestone, seagrass, and a coastline with a significant number of endemic species you will not meet anywhere else.

If you want warm water and hard coral, go north. If you want sea dragons, giant cuttlefish, artificial reefs made from government dredges, and a decommissioned warship, South Australia is worth the flight.

Where the diving is

Most of the state's 1.9 million people live in or around Adelaide, and the diving follows them. The east coast of Gulf St Vincent, running from the top of the gulf down past Adelaide, is the densest cluster of sites in the state.

Spencer Gulf and Gulf St Vincent are less rugged than the cliff coast along the Great Australian Bight and the western Eyre Peninsula. Kangaroo Island, Australia's third largest island, sits off the centre of the coast.

The Gulf St Vincent sites fall into three rough families. There are old shorelines: Broken Bottom and Leather Jacket Alley at 10 m, Mac's Ground with its 150 m ridge standing a metre off the seabed, Seacliffe Reef at 15 m. There are wrecks. And there are the artificial reefs, including tyre reefs at Grange, Glenelg and Port Noarlunga built from tyre modules ballasted with concrete.

The wrecks worth naming

Some of these ships were lost, and some were put down on purpose.

• Norma, a four-masted iron barque rammed at anchor in 1907, now in 14 m
• John Robb, sunk in 1910, maximum depth 20 m
• Star of Greece, a three-masted iron ship aground in an 1888 storm, in 6 m and reachable from shore
• Glenelg Dredge, the former SA government suction dredge South Australian, sunk in 1985, 20 m
• Glenelg Barge, a government sand hopper barge sunk in 1984, 20 m
• Port Stanvac Barges, three barges sunk in 1954 on a sandy bottom, 28 m
• H.A. Lumb, a steel tug put down by the Dive Industry Association of SA in 1994, 20 m
• Sea Wolf, a fishing trawler sunk by the Seawolves Dive Club in 2002, 20 m
• Fleurieu Artificial Reef, the ex-HMAS Hobart, an Adams class guided missile destroyer sunk in Yankalilla Bay in late 2002, 30 m

The Hobart is the headline act. A guided missile destroyer, sunk deliberately, sitting in 30 m of temperate water a short run off the Fleurieu Peninsula.

Port Noarlunga and the jetty dives

Port Noarlunga Reef is the one site most visitors manage without a boat. A 1.6 km reef, exposed at low tide, connected to shore by a 200 m jetty with a staircase down to the water. Maximum depth 20 m. The reef itself looks oddly man-made, like a collapsed wharf with the blocks shoved off each side, flat on top and drying by about half a metre.

The inshore side is well sheltered from swell. The offshore side near the jetty is a jumble of big blocky boulders with gaps, overhangs and crevices a metre or two deep, down to sand at about 9 m. Expect sparse kelp, crustose corallines, sponges and a fair variety of invertebrates. Street parking, public toilets, changing rooms and a fresh water shower at the jetty, according to the Wikivoyage diving guide.

Rapid Bay Jetty is the other must. Wood and steel columns, a partially collapsed old jetty, a tee at the end with pile dolphins, and roughly 12 m at the eastern end. A row of stakes in the sand guides you across the gap from the new jetty. The area at the tee is called the cathedral, and that is where the fish gather.

Rapid Bay is famous for leafy sea dragons, though the guide notes they seem less common than in the past. Weedy sea dragons turn up occasionally. Giant cuttlefish and bobtails too.

Further south on the Fleurieu, Second Valley headland gives you a steeply sloping rocky reef off a small promontory, sand at about 9 m, folded sedimentary rock and mixed kelp. Nearby Lasseter's Reef is a circular limestone outcrop about 50 m across on sand, ringed by seagrass, topping out around 3 m. The offshore side has more structure and more fish.

Seasons, water and the upwelling

The southern coast has a Mediterranean climate: mild wet winters, hot dry summers. The state's main temperature range runs from 29°C in January down to 15°C in July, with most of the southern rain falling in winter.

The thing that shapes the diving, though, is the Great South Australian Coastal Upwelling System. It is Australia's only deep-reaching coastal upwelling, pulling nutrient-rich water from depths beyond 300 m, and it stretches roughly 800 km from Ceduna in the eastern Great Australian Bight to Portland in Victoria. Upwelling events usually run December to April, driven by southeasterly winds, strongest off the southern tip of the Eyre Peninsula and more modest off south-western Kangaroo Island.

That cold, fertile water feeds the whole show. Juvenile southern bluefin tuna aggregate in the eastern Bight to feed on pilchards during the upwelling season. Sea lion, little penguin and seal colonies depend on it, as does a locally high abundance of blue whales and sharks. Calling it one of Australia's natural wonders is not an exaggeration.

Read that table and the shape of the year is clear. Water sits in the mid to high teens all twelve months, peaking around April and bottoming out in August. Wave heights build through winter and ease back over summer, which is also when the upwelling is running. The numbers come from Open-Meteo marine data.

Planning notes

The coast is split into coastal marine weather forecast areas, and the ones you will care about depend on where you are headed: Gulf St Vincent and Adelaide metropolitan waters for the Adelaide sites, Investigator Strait and South Central Coast for the Fleurieu. Forecasts come from the Bureau of Meteorology among others.

Most Gulf St Vincent sites are boat access only. The exceptions are the ones worth building a trip around on their own: Port Noarlunga Reef, Star of Greece, Hallett Cove, Glenelg Blocks, Second Valley, Lasseter's Reef and Rapid Bay Jetty. Horseshoe Reef and Port Noarlunga Reef both sit inside the Port Noarlunga Aquatic Reserve.

Adelaide is the obvious base. Festivals, fine wine, and a gulf full of wrecks within boat range.

When you get there, log it. Rapid Bay deserves a proper entry with your sea dragon photos if you find one, and the ex-HMAS Hobart deserves a depth and a rating. Drop them into Scubaspot and tag the buddy who talked you into the cold water.