Rosemary Island is one of the largest outer islands of the archipelago, with multiple reef types and sandy bays that provide sheltered anchorages. The Marine Life Network lists depths of 8–15 m with reef fish, colourful corals and invertebrates.

Its size means a lee shore is usually available, and the island is a turtle-nesting site. The sheltered bays make it a practical destination for a day's diving and snorkelling.

About the Dampier Archipelago: the archipelago's 42 islands off Dampier and Karratha hold one of the most diverse marine environments in Western Australia, with more than 650 recorded fish species, coral reefs, sponge gardens and seagrass meadows — all set against red Pilbara rock and the world's largest concentration of Aboriginal rock art on the Burrup Peninsula (Murujuga). It is heavily fished and boated by locals but lightly dived, and visiting divers usually go with Pilbara Dive and Tours or a local club. The combination of big tides, turbid inshore water and strong currents means the outer islands generally offer the best visibility and coral, and dive timing is planned around slack water.

Planning notes: Open Water certification is recommended and the site is rated intermediate. Expect to spend most of the dive around 12 m, with a maximum of about 15 m. Access is by boat only, and site selection depends on wind, swell and tide on the day. Currents are significant, so timing around the tide is part of the plan. Best season: April–October (outside cyclone season; best visibility in cooler months). Traffic: moderate (occasional charters/club dives).

From Karratha drive 20 km west to Dampier and launch at the Dampier public boat ramp (Hampton Harbour) or the Withnell Bay ramp on the Burrup Peninsula; both have trailer parking. Pilbara Dive and Tours (Karratha) runs small-group dive boats (maximum about six divers) with pickup from Karratha. The inner islands are 20–40 minutes from Dampier and the outer islands (Rosemary, Legendre, Delambre) 45–90 minutes depending on conditions.

There are no public moorings at most sites; boats anchor on sand. The archipelago's 42 islands are mostly within Murujuga National Park or nature reserves.

The Dampier Archipelago is sheltered from ocean swell by its islands, but the big Pilbara tides (spring ranges approaching 5 m) drive strong currents between the islands and control visibility; dive near slack water and plan around neap tides for the clearest water. Visibility is usually under 5 m in summer and up to 10–15 m in the cooler months, better at the outer islands. Strong easterly winds in winter and afternoon sea breezes chop up the crossings.

Cyclone season runs November–April and the Pilbara coast is Australia's most cyclone-prone; charters stop during warnings. Irukandji and box jellyfish can occur in the warmer months. Water temperature ranges from about 21 °C in July–August to 30 °C in January–February.

Choose the lee side, anchor in a sandy bay and swim out to the reef, following its edge at 10–14 m before returning shallower.

Pilbara Dive and Tours (Karratha) – Ultimate Dive Tour and Samson Wrecks tour (six-hour, max six divers), snorkel tours and private charters; otherwise private boats from Dampier. Lethal Adventures offers island boat tours.