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. Blue Line Lagoon is a circular reef with a drop-off, described by the Marine Life Network as having pelagic fish, reef sharks and grouper, with sea snakes common.

It marks the northern limit for charters based in the south, so it is usually the furthest point of a southern-based itinerary. The lagoon provides shelter, while the drop-off offers wall diving with pelagic encounters. Depths are not published; the figures given here are estimates.

The site suits Advanced divers comfortable with walls and current. 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 drop-off and follow it at 15-25 m with the current, scanning the blue for pelagics and sharks, then ascend onto the reef top.

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.