St Bees Island lies immediately east of Keswick Island across Egremont Passage, in the South Cumberland Islands National Park north-east of Mackay. It is a hilly, forested island best known ashore for its koala population, and its sheltered western bays carry fringing reefs that are dived by private boats and local clubs. The Marine Life Network's Mackay listing highlights two specific St Bees sites – the MV Cremer wreck near Grimston Point on the north coast, and 'Keswick Caves', crevices near the Cremer recently explored by a local club – and notes that the island's north coast is protected from all but south-easterly seas, with current along the cliff face.

More general fringing-reef diving around the island offers hard corals, reef fish such as coral trout and sweetlips, turtles and rays, in the modest visibility typical of the Mackay inshore islands. The dominant factor is the tide. Mackay's tidal range exceeds 6 m on springs, and Egremont Passage between Keswick and St Bees runs very strongly; dive at slack water on neap tides, anchor securely and keep a boat tender aboard.

St Bees has no public facilities or ferry, so it is a destination for private boats. Combined with Keswick's shore dives and the Cremer and Singapore wrecks, it offers a quiet island weekend of diving for self-sufficient divers from Mackay. Detailed site documentation for the general fringing reef is limited, and confidence is low-to-medium.

Anyone planning to land should check current national park and private-lease arrangements, as parts of the island have historically been under private lease.

Keswick and St Bees islands lie in the South Cumberland group roughly 30–35 km north-east of Mackay. A passenger ferry/barge service runs to Keswick Island from Mackay (about 1 hour), landing near Connie Bay; there are also light-aircraft flights. Private boats launch at Mackay Harbour or Mackay Marina (Mackay Harbour boat ramp, Slade Point) and cross in about 45–70 minutes depending on conditions, passing Slade Point and Flat-top Island.

Egremont Passage separates Keswick from St Bees and runs strong tides. There is no dive shop on the islands – bring all gear and fills from Mackay. St Bees is a national park island with no public ferry; reach it by private boat or charter, anchoring on sand in the sheltered western bays.

Mackay has the largest tidal range on the Great Barrier Reef coast (springs exceed 6 m), so tidal streams around the Cumberland Islands and in passages such as Egremont Passage are strong and visibility is closely tied to the tide – dive neap tides at slack water. The south-east trades (15–25 knots April–September) make crossings rough; the calmest windows are the build-up months (September–December) and winter lulls. Visibility is typically 5–12 m inshore, often worse after wind or wet-season rain (January–April) from the Pioneer River and local creeks.

Marine stinger season (box jellyfish and Irukandji) runs roughly November to May – wear a full-length stinger suit or wetsuit with hood and gloves, carry vinegar. Cyclone season November–April; tropical lows and cyclones can close sites and damage coral (TC Yasi 2011, TC Debbie 2017). Cyclone Hamish (2009) and Cyclone Debbie (2017) affected reefs in the region.

Anchor in a sheltered western bay, dive the fringing reef at 3–10 m at slack water keeping the reef on one side, and return to the boat before the tide runs.

No regular dive-charter operator currently runs scheduled trips in the Mackay region; divers use private boats, club trips (local dive clubs) and occasional charter boats from Mackay Marina. Nearest dive shops and fills in Mackay.