Corsair Rock is the ship-killer of Port Phillip Heads. The Scuba Doctor listing describes a submerged pinnacle about 6 m across with 3.5 m of water over its kelp-covered top, marking the south-western edge of Nepean Bank at the entrance. On the flood it shows as a swirl on the surface; on a strong ebb it breaks in short seas and white water. It was charted in October 1853 from the pilot cutter Corsair – a vessel that was itself wrecked on Nepean Reef in 1874 – and it has been putting ships ashore ever since: the Ontario and the French ship Marie both in 1853, the Eliza Ramsden in 1875 (she struck here and sank later inside the bay), the barque Craigburn, and most famously the P&O mail steamer RMS Australia, which ran onto the rock in fog in June 1904 and broke up; her plating lies in 3–4 m on the reef about 150 m to the north near Inner Corsair Wall.

Underwater the pinnacle rises from about 30 m on its seaward side, with gutters and ledges cut into its flanks and a flat, kelp-topped summit. The upper rock is scoured by the current and carries kelp and coralline algae rather than the sponge gardens of the walls, but the ledges below 10 m are heavily encrusted, southern rock lobster are numerous, and schools of sweep and old wives hold in the lee. Seals from the Point Nepean colony are frequent. Wreck debris – plating, fittings, ballast – is scattered in the gutters, most of it unattributable.

The dive is about position as much as scenery: being on the rock that shaped the Heads' maritime history, in water that is rarely still. It is dived only on the ebb slack in calm seas and is a club and private-boat site rather than a charter regular. Advanced divers with a competent skipper can circle the pinnacle in the 10–20 m band; the base at 30 m is for the experienced.

Boat access only. Charter boats leave from Portsea Pier (Dive Victoria Group, Bayplay) and Queenscliff Harbour (Dive Victoria Group, Red Boats); run time is about 12 minutes. Private boats launch at Sorrento Boat Ramp (Point Nepean Road, Sorrento, all-tide concrete ramp with trailer parking) or Queenscliff Boat Harbour ramp (Larkin Parade, Queenscliff), roughly 15 minutes to the site.

The site is at the entrance on the Nepean side; the rock shows as a swirl on the flood and as breaking white water on a strong ebb, and the kelp on its top is visible from the surface in calm water. Skippers must call Lonsdale VTS (Point Lonsdale) or listen to VHF 12 for shipping movements before dropping divers, and must carry a dive flag; anchoring is not permitted in the shipping channel, so a live boat and a drop-shot or drift float is the normal method.

It is dived only on slack water at the end of the ebb (SWE); on other tides the current across the site is unworkable. Slack water at Port Phillip Heads does not coincide with high or low water: it falls roughly three hours after high or low water at Point Lonsdale, and the usable window is typically 20–40 minutes. Use the Ports Victoria / Port of Melbourne slack-water predictions for The Heads (also republished by charter operators and WillyWeather 'Rip Bank') rather than a standard tide table.

Being outside or right at the entrance, it is exposed to Bass Strait swell: anything over about 1.5 m from the south-west or south, or a strong southerly/westerly wind against the ebb, produces standing waves in The Rip and charters cancel. Northerlies flatten the entrance and give the best days. Visibility is best in late summer and autumn (Feb–May) when the water is clear and the westerlies have eased; winter brings 11–13 °C water and bigger swells but often the clearest days after a run of settled weather.

Heavy rain has little effect this close to the ocean. The listing restricts it to slack water at the end of the ebb; it can only be approached in calm seas, because the rock breaks in any swell and the flood forms whirlpools over it.

Live-boat drop up-current of the rock at the ebb slack. Descend onto the kelp-covered top at 3.5 m and circle the pinnacle downward, checking the gutters at 8–15 m for wreckage and crayfish, then drop to the base at 20–30 m on the seaward side if conditions allow. Ascend the rock to the top, launch a DSMB and drift clear for pick-up before the flood sets in.

No regular scheduled charter – dived from private and club boats (VSAG, Power House Underwater Club, Scuba Diving Melbourne) and occasionally by Dive Victoria Group or Red Boats when a group requests it.