Sea Pen Point, at the north-eastern tip of Rocky Cape, is rated 7 out of 10 by the Marine Life Network guide and named for its unusual population of sea pens in shallow water - they are typically found below 30 m. A series of reefs and bommies line the southern shore, the bottom drops to sand 50-75 m offshore in 10-16 m, and the extreme end slopes steeply to 18 m. Sponges, gorgonians, sea daisies and nudibranchs cover the rock walls, schools of barber perch hang over them, red mullet are common on the sand, and bullseyes and southern hulafish shelter in cave shadows. The guide notes that it is heavily tide-affected and heavily dived, with pressure showing on the crayfish and abalone. 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.

Rocky Cape National Park is off the Bass Highway between Wynyard and Stanley, about 30 minutes west of Wynyard and 1 hr 15 min west of Devonport (Spirit of Tasmania terminal). A Parks Pass is required. Boats launch at the Rocky Cape boat ramps (the western ramp at Mary Ann Bay suits boats up to about 5 m), Sisters Beach or Boat Harbour; ramps are shallow and weedy at low tide, so a 4WD tow vehicle helps.

Sea Pen Point is a short run from the Rocky Cape ramp to the north-eastern tip of the cape, south of Cave Bay.

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.

Heavily tide-affected: calmest about 3-3.5 hours after low water. Relatively protected from easterly weather.

Drop at the reef edge, explore the bommies and boulders along the southern shoreline, and visit the sand edge at 10-16 m for the sea pens; the extreme end slopes to 18 m. 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.