Troubridge Island is a low, shifting sand island on Troubridge Shoal at the mouth of Gulf St Vincent, crowned by a cast-iron lighthouse prefabricated in England and first lit in 1856. The shoal was a notorious hazard for ships entering the gulf bound for Port Adelaide, and the Marine Life Society of SA notes that Marion Reef, south of the island, holds the remains of six of the earliest sailing vessels wrecked in the colony: the Dart and Parsee (1838), Sultana (1849), Marion (1851), Charles Carter (1854) and Iron King (1873). The Marion was a 919-ton wooden ship carrying emigrants from Plymouth; she struck the reef in slight fog and calm seas in 1851 and all passengers were landed safely.

Her anchor was raised in the 1970s and is displayed in Edithburgh. The Iron King, an 871-ton iron ship built in 1867 and about 60 m long, was wrecked on the shoal in 1873 on a voyage from London to Adelaide with 32 passengers, all of whom were saved. Little remains of most of these ships beyond scattered iron, ballast and fittings among the reef, but for wreck enthusiasts they are among the oldest shipwreck sites in the state.

The reefs themselves support diverse temperate marine life – sponges, gorgonians and ascidians on the harder bottom, and schools of sweep, trevally, old wives and leatherjackets – and the island hosts a little penguin colony and seabirds. This is not a casual dive. The shoal is very exposed, current-swept and dangerous to boats, and the Marine Life Society describes it as rarely dived by all but experienced enthusiasts.

It suits experienced boat divers with maritime history interests and local knowledge, on the calmest days of the year.

Launch at Edithburgh and run about 8 km south-east to Troubridge Island (20-25 minutes); Marion Reef lies to the south of the island on the shoal. The shoal is shallow, uncharted in detail and dangerous to boats – approach slowly on a rising tide with a sounder and local knowledge. Landing on Troubridge Island (a conservation park with a little penguin colony) is by arrangement only; historically access to the island has been via tours from Edithburgh.

Troubridge Shoal sits at the junction of Gulf St Vincent and Investigator Strait and is very exposed and current-affected. Strong tidal streams run across the shoal – dive neaps and slack water only – and southerly or south-westerly swell breaks on the reef edges. Wind against tide produces steep, dangerous seas for small boats.

Only attempt it in genuinely settled conditions with winds under about 10 knots and low swell, and with a boat tender following divers' SMBs.

Anchor in sand off the reef edge at slack water, descend and follow the reef edge looking for scattered iron and ballast of the old wrecks; keep the dive short, carry an SMB and be ready for pick-up drift if the tide turns.

No operator; very occasionally dived by experienced locals and maritime archaeology groups with their own boats. The Marine Life Society notes it is rarely dived by all but experienced enthusiasts.