The Swains are the most remote reefs of the southern Great Barrier Reef: around 1,000 reefs spread across the outer shelf, separated by channels 30-80 m deep, with the continental slope dropping to about 1,000 m just offshore. Diving is by liveaboard or private yacht only, and divers must be self-sufficient, with no assistance nearby. Gannet Cay is one of the nine small cays of Swain Reefs National Park, a seabird rookery of about 1.7 hectares that is closed to visitors year-round.

The cays of the Swains form one of six core seabird breeding areas in Queensland, supporting masked and brown boobies, lesser frigatebirds, noddies and terns, and they are also significant for loggerhead and green turtles. Underwater, the Marine Life Network describes coral pinnacles off the cay's southern side bursting with fish, including sharks and manta rays, at 7-35 m. The pinnacles rise from deeper water, drawing reef fish and pelagics, and the seabird colony above hints at the productivity of the surrounding water.

Divers must not land on the cay, and boats should keep noise and lights low near it, particularly during nesting. Aircraft must stay above 1,500 feet. The site suits Advanced divers comfortable with depth and current, as part of a Swains liveaboard trip.

The Swains lie within the Great Barrier Reef Marine Park under a mix of zones, and the nine vegetated cays of Swain Reefs National Park are among Queensland's most important seabird breeding sites and significant for endangered loggerhead turtles; most are closed to landing year-round. Anchor only on sand, check the zoning map before fishing, and carry oxygen, a satellite communicator and a detailed evacuation plan, since aeromedical retrieval to Mackay and a chamber in Townsville takes many hours.

The Swain Reefs are the most remote part of the southern Great Barrier Reef, roughly 200-250 km offshore. Liveaboards depart Gladstone Marina or Rosslyn Bay Harbour (Yeppoon) in the evening and steam overnight across the 50-mile-wide Capricorn Channel, a passage of 12-15 hours in good weather; trips run 6-10 nights. Private yachts should carry detailed charts (reefs are separated by 30-80 m deep channels) and plan for no outside assistance.

Once in the Swains, liveaboards move between reefs by day and anchor in the lee of a reef or cay at night; this site is visited as part of a multi-day itinerary.

The Swains are fully exposed. South-east trade winds of 20-30 knots, common June-August, make the Capricorn Channel crossing rough and restrict diving to the lee (north-western) sides of reefs; charter operators generally shelter behind reefs at anchor in 20-25 knots. Best windows are the calm spells of September-December and March-May.

Cyclones can reach the area November-April. Tidal currents run hard through the channels between reefs, so outer-edge sites are dived as drifts near slack water; lagoon and lee sites work in most tides.

Tender to the pinnacles off the southern side, descend the largest to 20-25 m and circle it, then move between pinnacles at a shallower level, watching for mantas and sharks. Surface under an SMB well away from the cay.

No day operator. Kanimbla Charters runs 7- and 10-day dive and fishing charters to the Swains aboard a 25 m catamaran (all tanks and weights supplied); Eastern Voyager runs 6-7 night trips from Gladstone mainly for fishing, snorkelling and spearfishing. Most diving here is from private yachts and occasional expedition charters.