The reef below O'Hara Bluff is a gently sloping kelp-covered reef between Pirates Bay and Fortescue Bay. The Marine Life Network guide describes it as O'Hara Reef in 10-20 m, with a kelp garden and crayfish. Historically this stretch of coast supported dense forests of giant kelp (Macrocystis pyrifera), which grows from the reef to the surface in columns and forms a floating canopy.

Those forests have almost disappeared: scientists estimate a 95 per cent loss of giant kelp on Tasmania's east coast since the 1950s, driven by warming water carried south by a strengthening East Australian Current, low nutrients and overgrazing by the long-spined sea urchin. The Tasman Peninsula has become a focus of efforts to protect the remnants and restore what has been lost. Researchers from the Institute for Marine and Antarctic Studies have grown giant kelp from seed and grafted it onto reef here, producing a 3,500 square metre kelp field after 18 months that survived a marine heatwave, and Eaglehawk Dive Centre now runs a winter internship in which divers learn restoration techniques.

What divers see at O'Hara Bluff therefore varies from year to year. In good years the canopy is thick enough to swim through the stipes with sunlight filtering down, and the understorey shelters weedy seadragons, wrasse, leatherjackets, magpie morwong and rock lobster. In poorer years the reef is dominated by shorter brown algae such as crayweed and Ecklonia, still pleasant but without the cathedral effect.

Fur seals from nearby colonies often join divers in the shallows, playing in the kelp. It is an easy, shallow dive suitable for all levels. Ask the dive centre about the current state of the kelp and respect restoration plots.

Boat access only. Eaglehawk Dive Centre and Go Dive Tasmania run from the Pirates Bay jetty at Eaglehawk Neck; O'Hara Bluff lies south of Waterfall Bay, about 15 minutes by boat. Private boats can launch at Pirates Bay or at the Fortescue Bay ramp (12 km gravel road from the Arthur Highway near Taranna; national park pass required) and run north for about 10 minutes.

The Tasman Peninsula faces the full Southern Ocean and Tasman Sea. South-westerly to south-easterly groundswell and strong easterlies are what cancel trips: operators generally need westerly or north-westerly winds with swell under about 1.5-2 m for the open-coast sites. Prolonged southerly swell pushes surge deep into the caves and gutters and churns visibility; after a big blow allow two or three days to settle.

Late summer and autumn (February-May) bring the most settled windows and the clearest water; winter can be glassy between fronts but fronts arrive every few days. O'Hara Bluff faces east; westerly winds make it flat. Marine heatwaves in late summer can thin the kelp canopy.

Descend on the outer edge of the reef at about 15-18 m, then work inshore up the slope through the kelp stipes, looking in the understorey for seadragons and crayfish. Finish in 5-8 m beneath the canopy and ascend in a clear patch away from the fronds.

Eaglehawk Dive Centre – kelp and seal dives when giant kelp is present and as part of restoration work; Go Dive Tasmania – kelp dives on Tasman Peninsula boat days.