Fortescue Bay is a broad, sandy bay inside Tasman National Park, ringed by forest and campsites and sheltered by Cape Hauy. It is the easiest shore diving on the Tasman Peninsula, and it holds one of the few remaining stands of giant kelp on Tasmania's east coast. The Divers Alert Network's Tasmania feature describes the Fortescue Bay kelp forest at about 12 m as the last remaining refugium on the peninsula after a 90-95 per cent loss of these forests, while dive-travel writers describe kelp rising from white sand to heights of up to 15 m.

The Marine Life Network guide records Macrocystis columns interspersed with sand and rock patches near the boat ramp in 5-20 m. Swimming among the stipes, with light filtering through the canopy, is the reason to come. The forest shelters Johnston's weedfish, banded stingarees, Australian swellsharks, short-tail stingrays, long-snout boarfish, weedy seadragons and draughtboard sharks, and fur seals sometimes visit.

The rocky reefs along both shores add wrasse, leatherjackets, magpie morwong and rock lobster, and the sand is home to rays and flathead. Go Dive Tasmania lists the bay at 3-12 m for all certified divers, noting clear water and abundant fish. The bay has an industrial past – timber milling and a fish works at Canoe Bay operated into the 1970s – and the wreck of the dredge William Pitt, scuttled as a breakwater in 1953, lies in the mouth of Canoe Bay a short walk away.

With camping on the beach, Fortescue Bay makes an excellent base for a diving weekend: shore dives at dawn and dusk, and boat dives to the Hippolytes and the Nord from the ramp. Because the kelp is fragile and slow to recover, divers should avoid pulling on stipes and keep fins clear of holdfasts.

From Hobart take the Arthur Highway to Taranna, then turn onto Fortescue Road just south of the town and follow 12 km of gravel road to the bay (about 1 hour 30 minutes from Hobart). A Tasmanian national park entry pass is required. There is a camping ground, day-use area and boat ramp at the northern end of the beach near the track to Cape Hauy; park near the ramp or beach and walk a short distance to the water.

The kelp forest is described as close to the boat ramp. Boats can also reach the bay from Pirates Bay in about 30 minutes.

Fortescue Bay opens to the east-north-east but is well sheltered by Cape Hauy to the south and the coast to the north, making it one of the most reliably calm places on the peninsula. Strong easterly or north-easterly winds push swell into the bay and cloud the water; southerly groundswell occasionally wraps in. Westerly and southerly winds leave it flat.

Marine heatwaves in late summer stress the giant kelp and may thin the canopy. Heavy rain has little effect beyond a slight tannin layer near the creek at the southern end.

Enter near the boat ramp, surface swim or descend along the rocky northern shore to the kelp at 8-15 m, and explore the stipes and sand patches. Turn at half air and return along the reef edge, keeping well clear of the ramp.

Go Dive Tasmania – Fortescue Bay listed as a shore site (3-12 m, all certified divers); Eaglehawk Dive Centre – giant kelp dives and restoration internship work in the area; club weekends from Hobart.