The Gap is one of the best-known lookouts in Dhilba Guuranda-Innes National Park, a deep cleft in the limestone cliffs where the Southern Ocean surges in, and the reef below it is one of the park's showpiece dives. The Marine Life Society describes a boulder reef with caves, including one more than 50 m long, in 10-15 m, while The Dive Shack and visitor information emphasise vertical and sloping walls, swim-throughs and abundant fish. The underwater landscape is a jumble of huge limestone blocks, gutters and overhangs, with a cave system that tunnels into the reef.

The walls and roofs are coated in sponges, gorgonians, ascidians and soft corals, and the fish life is rich: western blue groper, southern blue devils, sweep, morwong, boarfish, bullseyes, nannygai and wobbegong sharks, with crayfish in the cracks. The caves are the highlight and the main hazard. The Marine Life Society warns that it is easy to become disoriented because the tunnels lead almost in a complete circle, and a torch is essential.

Surge in the caves can be dangerous, so the site is only dived in low swell. Hole in the Wall – a 40 m-wide cave running about 70 m under the reef – lies 100 m away and can be combined on the same tank. This is an advanced dive for experienced divers with good buoyancy and navigation, visiting by boat on the calmest days.

When it works, The Gap delivers the full southern Australian limestone-reef experience: clear water, big blue groper, cave swim-throughs and colourful invertebrate walls.

From Adelaide drive about 300 km (3.5 hours) via Port Wakefield, Minlaton and Warooka to Marion Bay; Dhilba Guuranda-Innes National Park begins just west of the township (vehicle entry fee, pay online or at the Stenhouse Bay visitor centre). Boat access only: launch at the Marion Bay ramp (about 25-30 minutes west past Cape Spencer) or, in settled weather, at the Pondalowie Bay ramp inside the park on the west coast (about 15 minutes). The Gap is a well-known clifftop lookout reached by the park road, which helps identify the site from the water.

Hole in the Wall lies about 100 m away and can be combined on the same tank.

The southern tip of Yorke Peninsula faces the open Southern Ocean and Investigator Strait, so south-westerly and southerly swell is the controlling factor: most exposed sites need swell under about 1.5 m and are best after several days of northerly or north-easterly winds, which blow offshore along this coast. Winter cold fronts bring big south-westerly swell and strong winds and can close the area for days. Summer afternoon south-easterly sea breezes make boat runs rough, so launch early.

Visibility is typically 10-20 m in settled conditions but drops sharply with swell-driven surge. The Gap faces south into the swell and is only dived in low swell; visitor information notes visibility is strongly affected by swell.

Drop onto the boulder reef at 10-12 m, explore the walls and gutters, then enter the main cave with a torch (only with good buoyancy and low surge), noting that the tunnels curve almost in a circle. Exit to open water and, with gas in hand, swim 100 m to Hole in the Wall before ascending.

Caves are open swim-throughs, not true cave penetrations, but they are long and dark; carry a primary and backup torch.

No resident operator; visited on Adelaide shop and club boat trips to Innes and by private boats from Marion Bay and Pondalowie Bay.