Grey Crags at Western Bay is rated 6 out of 10 by the Marine Life Network guide. Quartz ridges run parallel to the shore from Seal Rock to Rocky Bay; about 50 m offshore the bottom drops quickly from 5 m to 10 m, then to jagged ridges at 15 m, forming a maze of sand-filled gutters, rock walls and bommies. Lace bryozoa, cup corals, nudibranchs and tunicates fill the crevices, large schools of barber perch hang on the rock faces, and trumpeter, sea sweep, zebrafish, long-finned pike, magpie perch, rock cod and wrasse are common. The guide advises good air monitoring and navigation for the longer current-assisted swims. 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.

Western Bay is just west of Boat Harbour Beach, 15-20 minutes west of Wynyard. Park at the convenient Western Bay parking area, then walk down the embankment or swim at high tide over the rocky flats. The site extends west to Rocky Bay, allowing a one-way drift with a car shuffle between the two parking areas.

Check the tide direction first: the current runs east-west on the rising tide and west-east on the ebb.

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.

Dangerous in rough weather; a slight current runs east-west on the rising tide and west-east on the ebb.

Swim out 50 m to the drop from 5 m to 10 m, then explore the maze of sand-filled gutters, walls and bommies at 15 m; experienced divers can follow the current for a longer one-way dive. Turn at a third of gas to allow for the swim back and a possibly harder exit, finish with a 3-minute safety stop at about 5 m if the depth allows, and exit where you entered unless an alternative exit has been checked beforehand.

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.