The southern side of Cook Island Aquatic Reserve receives a fraction of the diver traffic of the sheltered Northern Ledge a short distance away, mainly because it takes more of the prevailing swell coming up from the south-east and is consequently run less consistently by the Gold Coast and Tweed Heads charter operators. For divers who do get out here on a settled day, though, the reward is a noticeably different, slightly more current-affected character to the same small volcanic island, with deeper ledges stepping down to 24 m and a correspondingly better chance of pelagic fish moving through on the current. The underlying rock and coral structure is continuous with the rest of the island, so the same rich covering of sponges, gorgonian fans and hard and soft corals found at the Northern Ledge continues around to this side, but the greater current exposure concentrates schooling kingfish, trevally and the occasional bronze whaler shark in a way the more sheltered side rarely does.

Turtles remain common, and grey nurse sharks are seen here through the cooler months in the same pattern as elsewhere around the island, sometimes in the deeper gutters where the current provides some relief. Because this side of Cook Island sees more swell and current than the Northern Ledge, it is generally reserved for divers with a little more experience, and operators will typically default to the sheltered side unless conditions are unusually calm or a group specifically requests the change of scenery. For repeat visitors who already know the Northern Ledge well, though, the Southern Aquarium — as local divers sometimes call it, for the sheer density of fish that gathers here on a good current day — offers a worthwhile variation within the same easy 15-minute boat trip from the mainland.

Boat access only, from Kirra or Currumbin on the Gold Coast side or by private boat from Fingal Head/Tweed Heads, continuing around to the southern side of the island rather than stopping at the more commonly dived Northern Ledge.

The Tweed/Coolangatta coast is exposed to south-easterly and easterly swell year-round; charters generally want under 1.5 m swell and light winds. Visibility on the inshore reefs drops for days after heavy rain flushes the Tweed River. Winter (June–September) gives the clearest, calmest water and the grey nurse shark season; summer is warmer with more current and better odds of leopard sharks and mantas.

The southern side of Cook Island takes more of the prevailing swell than the sheltered Northern Ledge, so it is run less often and only when conditions are genuinely settled.

Descend the mooring onto the southern ledges at 10-14 m, work down to the deeper outcrops at 20-24 m watching for pelagics and grey nurse sharks moving past in the current, and shallow up over the reef for the ascent.

Gold Coast Dive Adventures and Kirra Dive will run the southern side of Cook Island as a variation from the Northern Ledge when conditions and group experience suit.