Cape Spencer is the southern tip of Yorke Peninsula, where the lighthouse stands above cliffs that drop into the meeting of Investigator Strait and the entrance to Spencer Gulf. The Marine Life Society describes the dive below the lighthouse as vertical drop-offs, undercuts, holes and caves in about 10 m, with caves that cut right through the point and blowholes 8-10 m long. The limestone here is sculpted by constant swell, producing a dramatic underwater landscape of walls and tunnels.

The walls carry sponges, gorgonians and ascidians in the sheltered undercuts, while kelp dominates the tops. Fish life reflects the exposure: western blue groper, sweep, silver drummer, old wives, magpie perch and blue devils, with seals from the nearby Althorpe and Haystack colonies passing through. The swim-throughs are the highlight but are only safe in very calm weather – surge through a blowhole can be violent.

Because the cape faces the open ocean on multiple sides, suitable days are uncommon, and most boats heading this way continue to Haystack Reef or Althorpe Island instead. Cape Spencer suits experienced divers with a boat who happen upon a rare flat day. It is a spectacular setting and a worthwhile addition to an Innes trip when conditions allow.

Skippers should keep the boat well off the cliffs, as rebound swell makes the water confused close in, and divers should be ready for a live-boat pick-up rather than returning to an anchor. A torch helps in the undercuts and tunnels, where the colour of the sponges and gorgonians only shows under artificial light. Seals are a bonus rather than a certainty here, but when they appear they tend to be curious and will follow divers along the wall.

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). Launch at Marion Bay and run west past Stenhouse Bay to Cape Spencer (about 25 minutes). The lighthouse is visible from well offshore.

Shore access down the cliffs is not practical for divers; the lighthouse car park is reached by the park road and gives a good view of sea conditions before committing.

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. Cape Spencer is the most exposed point on the peninsula; the Marine Life Society advises the swim-throughs only be attempted in very calm weather.

Drop in on the sheltered side of the point, follow the vertical walls and undercuts at 8-10 m, and explore the caves and blowholes that cut through the point only if surge is minimal.

No operator; occasionally dived by private boats and Adelaide clubs.