Betsey Island is the largest island at the mouth of the Derwent, lying off South Arm where the river meets Storm Bay, with Little Betsey Island close by to the east. It is Hobart's closest 'ocean' island dive, and its reefs are influenced by cleaner Storm Bay water more than the sites upriver. The reef consists of dolerite boulders and ledges covered in kelp and brown algae in the shallows, giving way to sponge-covered rock and sand in deeper water.

Typical fish include blue-throated and purple wrasse, leatherjackets, bastard trumpeter, magpie and banded morwong, and weedy seadragons are found in the weed. Rock lobster shelter in the crevices, and fur seals from Storm Bay sometimes visit. The island's main diving attraction, however, lies to the west of Little Betsey, where Hobart's ships' graveyard holds a dozen scuttled vessels and bridge sections.

Many club trips combine a wreck dive in the graveyard with a reef dive on the island. Betsey Island is a nature reserve and an important seabird breeding site; landing is not permitted. Visibility is variable: after a dry spell and on an incoming tide it can exceed 10 m, but Derwent floods and southerly swell quickly reduce it.

The island suits Open Water divers with some cold-water experience on its sheltered side, and Advanced divers on its exposed seaward reefs.

South Arm is about 35 km (40 minutes) south-east of Hobart: cross the Tasman Bridge, follow the South Arm Highway (B33) through Rokeby and Lauderdale to South Arm. The main boat ramp is at South Arm (Blessington Street/Harbour area) with trailer parking and toilets; Lauderdale (Doran's Road) is an alternative ramp further north. Betsey Island lies about 5-6 km south-east of the South Arm ramp across the Derwent mouth, 15-20 minutes by boat.

Choose the lee side according to wind and swell and anchor in sand clear of the reef. The island is a nature reserve with nesting seabirds; do not land.

The South Arm peninsula and Iron Pot sit at the junction of the Derwent and Storm Bay. Western shores and the lee sides of the islands are sheltered from easterlies; eastern and southern faces are exposed to southerly swell rolling in from Storm Bay. Strong southerly winds and swell close the exposed sites; northerly and light westerly winds give the best conditions.

Visibility varies widely, from 3 m after Derwent floods to 15 m in clear marine water after a dry spell and a flood tide. Afternoon sea breezes can make the boat ride back choppy. The seaward side is exposed to Storm Bay swell; in southerlies the northern and western sides give shelter.

Anchor on the lee side, descend to the reef edge at 12-18 m and follow the reef along the island, checking crevices for crayfish and the kelp for seadragons. Rise through the kelp for the safety stop.

No scheduled operator trips; Hobart club boats and private boats from South Arm; Go Dive Tasmania and The Dive Shop Hobart can advise.