Simmos Reef at Shag Rock is described by Michael McFadyen as a very good dive site. The bommie is shaped like a figure 6 with its top pointing north, rising from about 25 m of sand to about 4 m, with more than 20 m on the northern side and kelp on the upper sections. He recorded red sponges, lace coral, bryozoans and gorgonians, lemon-coloured sea pens, dumpling squid and cuttlefish, red morwong, seapike, leatherjackets, numerous small long-snouted boarfish and several starfish species, with 25 m visibility and 18.7 °C water in February.

The Marine Life Network guide describes what appears to be the same site as Elephant Shark (Shag) Rock, rated 5.5 out of 10: an arc of rock spires with bommies rising almost sheer from 10-18 m, large cup sponges and sea fans, and magpie perch, wrasse, leatherjackets, banded morwong, rainbowfish, scalyfin, barber perch and schools of bastard trumpeter and long-finned pike. Local divers typically combine two or three Rocky Cape and Sisters Island sites in a day, choosing the side of the cape that suits the wind and the state of the tide.

The Rocky Cape and Sisters Island area is, in the Leven Scuba Club's words, the best collection of dive sites on the north-west coast. The quartzite of Rocky Cape - some of the oldest rock in Tasmania - forms gutters, ledges, caves and bommies, and the tidal flow along Bass Strait keeps sponges, gorgonians, sea pens and bryozoans well fed. Michael McFadyen, diving here in February 2008, recorded 18-19 °C water and 20-25 m visibility, figures that surprise divers used to the east coast.

Rocky Cape National Park also protects Aboriginal cave sites and coastal heath above the water, and a Parks Pass is required for vehicles. The nearest hyperbaric chamber is the Royal Hobart Hospital, about four hours' drive away, so conservative profiles and oxygen on the boat are essential.

Sisters Beach is a small village in Rocky Cape National Park, off the Bass Highway about 25 minutes west of Wynyard (1 hr 15 min from Devonport). The Sisters Beach boat ramp is the launch point for Sisters Island (about 3-5 km); Boat Harbour ramp further east is an alternative. Ramps are shallow at low tide.

Shag Rock is about 3.6 km from the Sisters Beach ramp (McFadyen: 40 54 41.8 S, 145 35 21.1 E); the Marine Life Network guide places Elephant Shark (Shag) Rock 500 m north-north-east of the Glory Hole point rocks.

The north-west coast faces Bass Strait: north-westerly to north-easterly winds and westerly swell make it rough, while southerly winds blow offshore and give flat seas - light airs and sunny days are relatively common from January to April. Easterly gales are common in the warmer months and close the eastern faces of Rocky Cape and Sisters Island. Tidal streams run along the coast (eastward on the flood), and several boat sites are only comfortable around slack water.

Noticeable but generally manageable current; the boat operator should monitor divers.

Descend to the sand at about 25 m, travel anticlockwise around the eastern side, through the central gully and along the northern side, returning to the start while ascending gradually to 5 m for the safety stop (McFadyen's route). Turn the dive at half gas (thirds where current or overhead sections are involved), deploy an SMB before surfacing for the live-boat pick-up, and complete a 3-minute safety stop at about 5 m where depth allows.

No confirmed commercial dive operator on the north-west coast as of 2026 (older guides mention dive services in Wynyard and Devonport - verify). Sites are dived mainly by the Leven Scuba Club (Ulverstone), other north-west club members and visiting divers with their own boats; Tasmanian Divers (Launceston) runs occasional northern trips.