Sloping Island is a 117-hectare nature reserve off the north-western Tasman Peninsula, part of the Sloping Island group that includes the adjacent Sloping Reef. Go Dive Tasmania lists it as one of the few diveable giant kelp forests near Hobart, in 5-12 m and suitable for all certified divers, with seahorses and schooling fish. That makes it an important site: giant kelp has declined by around 95 per cent on Tasmania's east coast since the mid-twentieth century, and stands close enough to Hobart for a half-day trip are rare.

Diving through a healthy Macrocystis forest is a special experience, with golden light filtering through the canopy and fish moving among the stipes. Big-belly seahorses cling to the kelp and understorey algae, while schools of bastard trumpeter, wrasse, leatherjackets and juvenile fish shelter in the forest. The older Marine Life Network guide describes a reef about 100 m long and 50 m wide on the south side of the island in 4-8 m, attractive with reef fish and colourful sponge growths, and notes shallow reefs between Sloping Island and the mainland.

The island has ruins of early settlement foundations ashore, though little remains. The shallow depth means long, relaxed dives suitable for beginners and photographers. Kelp condition varies with summer water temperatures, so ask the operator about its current state.

Treat the kelp gently – avoid pulling on stipes or kicking holdfasts – and never land on the island, which protects nesting seabirds. Go Dive Tasmania's guided trips remove the logistics for visitors without their own boat.

Boat access. Launch at the Pipe Clay Lagoon area, Dunalley or Primrose Sands; the Marine Life Network guide suggests Pipe Clay Lagoon or Dunalley. From Dunalley (Arthur Highway, about 55 km from Hobart) the run through Norfolk Bay to Sloping Island is roughly 20-30 minutes.

Go Dive Tasmania runs guided boat dives here. Anchor in sand on the south side of the island clear of the kelp; the island is a nature reserve, so do not land.

Frederick Henry Bay is very exposed to strong southerly winds and swell, which the Marine Life Network notes can make it rough; a calm day is required. The south side of Sloping Island offers some shelter from swell. Northerly and westerly winds are usually fine.

Summer marine heatwaves can thin the giant kelp canopy, and visibility falls after strong southerlies stir up the bay.

Descend on the south side at the kelp edge, swim slowly through the stipes at 6-10 m looking for seahorses in the understorey, and loop back to the boat along the reef.

Go Dive Tasmania – Sloping Island Kelp Forest (5-12 m, all certified divers) on its boat dive program; private boats from Dunalley and Primrose Sands.