The Ardrossan Barge exists because of the Zanoni. When the historic barque was declared a protected wreck in 1983, fishers and divers lost access to the only substantial structure in that part of Gulf St Vincent, so in 1984 an alternative reef was created nearby by scuttling an old steel hopper barge. The vessel was the No. 5 dumb hopper barge, built in Sydney in 1911, 42.6 m (140 ft) long and 7.9 m (26 ft) wide, with nine compartments and bottom-opening hopper doors that once dumped dredge spoil.

Six 30 m steel pipes were placed inside the hull to add habitat. The barge sits upright on a sandy bottom in about 17-20 m, roughly one nautical mile south of the Zanoni. After four decades on the seabed it is heavily encrusted with sponges, ascidians and algae, and it functions exactly as intended: a fish magnet in otherwise featureless gulf.

Snapper, mulloway, Australian salmon, tommy ruff, leatherjackets and old wives school around it, and cuttlefish and crabs shelter in the compartments and pipe ends. The structure is simple and easy to navigate – a long rectangular hull with open compartments – but its depth, mid-gulf location and variable visibility put it in advanced territory. The compartments invite a look from above, but full penetration is not recommended because of silt and corrosion.

Most divers visit the barge as a second dive after a permitted Zanoni dive, or as an alternative when a Zanoni permit is not in hand. It is also popular with fishers, so expect boats and lines on weekends. It suits divers who enjoy artificial reefs and fish life and have their own transport across the gulf.

Launch at Ardrossan and run about 10-11 nautical miles south-east across the gulf (40-50 minutes), or cross from North Haven/Outer Harbor in Adelaide (about an hour). The barge lies roughly 1 nautical mile south of the Zanoni, outside the Zanoni's Protected Zone, and no permit is needed. Use GPS to find it and anchor on the sand beside the barge rather than on it.

Like the Zanoni, the barge is in open mid-gulf water with no shelter; plan for winds under about 12 knots and avoid days with south-westerly fronts or strong afternoon sea breezes. Current is usually mild but stronger on spring tides – neaps or the tide turn are best. Visibility varies from 5 m after wind to 15 m or more after a run of calm weather, since the silty bottom takes time to settle.

Descend the shot to the sand at about 20 m, circle the hull at depth, then rise to the deck (around 15-17 m) and explore the compartments and pipes from outside. Return to the shot and make a safety stop at 5 m.

Upright hull with nine open compartments; swim-through only with good buoyancy; no full penetration recommended.

No regular operator; dived by Adelaide clubs and private boats, often combined with a permit dive on the Zanoni. Popular with recreational fishers.