Keswick Island is a largely national-park island in the South Cumberland group with a small residential settlement and a ferry link to Mackay. The Marine Life Network describes Connie Bay, near the ferry terminal, as having 'a good coral garden which is popular as a beginners dive', with various coral patches on a silty bottom and visibility often around 10 m. These are easy, shallow dives suited to newer divers and snorkellers: coral patches of hard and soft corals, reef fish, and chances of turtles and rays, with whales passing in winter.

The silty bottom means good buoyancy control matters to keep visibility, and visibility itself depends on the tide – Mackay's large tidal range makes neap tides and slack water the best times. Nearby Wigton Island is also noted for coral gardens, and the Egremont Passage between Keswick and St Bees can offer drift snorkels for experienced swimmers, though currents there are strong. Keswick's appeal is its quiet: there is no dive shop, and divers need to bring their own gear and fills from Mackay, but the island offers independent shore diving from holiday accommodation.

Ferry timetables determine how long visitors have on the island, so day-trippers should plan dives around the ferry and the tide together.

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. Connie Bay is beside the ferry landing, so ferry passengers can walk straight to the water; Basil Bay is on the island's road network a short drive or walk from the settlement.

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.

Enter from the beach at Connie Bay at mid to high tide, fin out to the coral garden and explore the coral patches on the silty bottom at 3–8 m, returning along the shoreline before the tide turns.

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.