Cape Sonnerat is the south-eastern corner of Schouten Island and, according to the Marine Life Network guide, one of the best dive sites in Tasmania - it rates it 10 out of 10 and calls it breathtaking both above and below water. Depths are 30-40 m, and the guide rates it as a category 3-4 dive.

The topography is granite crevices and vertical formations, with a 4 m-wide cave that has a flue-like opening through which rays of light fall from the surface. Clouds of butterfly perch swarm around the rocks, cave-dwelling fish and crayfish shelter in the cracks, schools of fish pass through and seals are regular visitors. The guide notes that the most scenic caverns lie at decompression depths, which is what makes the dive both memorable and demanding.

Cape Sonnerat suits highly experienced divers, ideally with technical training, who can manage decompression in a remote location far from a chamber.

Schouten Island is part of Freycinet National Park. Its granite and dolerite peaks rise to over 400 m, and the island has no permanent residents, jetty or facilities apart from a Parks hut and campsite at Moreys Bay, so divers visit with self-contained boats. The island's sites were described in detail in the Marine Life Network regional guide, but few have been documented since; with no operator currently running trips, they are among the least-dived good sites in Tasmania.

A typical Schouten day combines one deeper dive on the exposed eastern or southern side with a shallow second dive in a sheltered bay, choosing sides according to the swell. Because the island is at least 45 minutes from the Coles Bay ramp and more than four hours by road and boat from the Royal Hobart Hospital chamber, conservative profiles, oxygen on board and a reliable weather window are essential.

Coles Bay is 30 minutes south of the Tasman Highway via the C302 (turn off 12 km south of Bicheno), about 2 hr 45 min from Hobart and 2 hr 30 min from Launceston. The main public boat ramp is at Coles Bay township; boats reaching the ocean side of the peninsula or Schouten Island need a seaworthy vessel, as the run around Cape Degerando or through Schouten Passage is exposed. Schouten Island lies across Schouten Passage from the southern tip of the Freycinet Peninsula, roughly 25-30 km (about 45-60 minutes in a fast boat) from the Coles Bay ramp via Promise Bay and the western shore of the peninsula.

Swansea's ramp on the western side of Great Oyster Bay is an alternative. Schouten Passage can run hard with wind against tide. Cape Sonnerat is at the south-eastern corner of the island, the furthest point from Coles Bay - allow about an hour each way.

Cape Sonnerat faces the open Tasman Sea to the south and east and is only diveable in calm, settled conditions with minimal swell. Westerly weather is best. Any real swell makes the cape dangerous for boat handling.

Rain washes tannin-stained runoff into Great Oyster Bay, and summer afternoons commonly bring a fresh north-easterly sea breeze, so plan dives for the morning.

Descend the granite face to the cave at 30-40 m, with its flue-like opening admitting light from above. Spend limited time at depth and ascend through the crevices and vertical formations, completing decompression or a long safety stop on the shallower granite.

Plan as a decompression dive on nitrox or trimix with a stage for deco; carry an SMB and redundant gas. Cavern penetration should only be done within daylight zone.

No dedicated dive operator at Coles Bay or Freycinet. Sites are dived from private boats and occasional club expeditions (Tas Uni Dive Club, TSDC, Launceston clubs); the Marine Life Network guide notes that Coles Bay is the usual starting point for Schouten Island charters. Cruise and kayak operators in Coles Bay do not offer scuba.