South Channel Fort is one of the strangest and most enjoyable dives in Port Phillip: a 0.7 ha artificial island built in the 1880s from granite boulders and concrete on a shoal in the South Channel, part of the defences that guarded the entrance to Melbourne with disappearing guns and a controlled electric minefield. Above water it is a labyrinth of tunnels, magazines and gun emplacements managed by Parks Victoria; below, the tumbled granite blocks that armour its slopes have become a reef of caves, hideaways, caverns and tunnels descending to about 12 m, with seagrass and sand beyond and a jetty on the western side that reaches the bottom at 12 m. The boulders are covered in kelp and colourful sponges, and the gaps between them shelter an unusually rich cast for the bay: giant Australian cuttlefish, southern keeled and Maori octopus, decorator crabs, bluespotted goatfish, southern blue devils in the darker holes – rare this far inside the Heads – and smooth stingrays on the sand.

Australian fur seals haul out on the fort's rocks and often join divers, and little penguins nest on the island. The dive can be run as a circumnavigation at about 5 m, as a deeper exploration of the jetty and boulder slope, or as a snorkel on the lee side. The complications are tide and traffic: the South Channel is the main shipping lane into Melbourne, its currents run hard on the flood and ebb, and the fort's position guarantees a sheltered side but also a swept one, so the dive is planned around slack water and the lee.

Landing on the island is restricted and visitors stay in the water. South Channel Fort suits Open Water divers and above on a charter or a competent private boat, photographers wanting seals, cuttlefish and granite caves, and snorkellers on a calm day.

Launch at Rye or Sorrento boat ramps and run 10–15 minutes north across the South Channel to the island; charters from Portsea and Queenscliff include it on bay days. Because the island is surrounded by shipping and tidal channels, anchor on whichever side is in the lee and outside the marked channel, and descend beside the granite boulders; the western jetty reaches sand at 12 m and is the usual deep entry. Landing on the island is restricted; divers stay in the water.

Kayak divers occasionally paddle from Rye in flat conditions, but the channel crossing is exposed.

The Scuba Doctor lists it as diveable in south-westerly conditions on either the flood or the ebb, and notes that the island's position means there is always a sheltered side; in practice divers work the lee and avoid the channel face where the tide runs. Currents through the South Channel are strong on the run and the shipping channel passes close, so plan for slack water where possible and never surface in the channel. Northerly winds make the anchorage lumpy but do not stop the dive.

Visibility is generally good – the South Channel carries clean water from the Heads – and best in winter.

Descend beside the western jetty to the sand at 12 m, work up through the granite boulders and kelp on the sheltered side at 5–8 m, exploring the caves and tunnels between the blocks, and circumnavigate at about 5 m if the tide allows, returning to the boat on the lee side. Ascend with a safety stop on the boulder slope and deploy an SMB.

Dive Victoria (Portsea/Queenscliff) and Bayplay (Portsea) include it on bay charter days and seal/snorkel trips; Red Boats (Portsea) private charters; Melbourne clubs and private boats from Rye and Sorrento.