Cerberus is the most important warship wreck in Victoria and the one nobody is allowed to dive. She was the world's first breastwork monitor – a low, heavily armoured, twin-turreted iron warship designed by Edward Reed and built by Palmers at Jarrow-on-Tyne, launched on 2 December 1868 and completed in 1870 for the colony of Victoria's own navy. Sixty-eight metres (225 ft) long with four 10-inch muzzle-loading guns in two turrets, she was the first ship to combine a central superstructure with turrets fore and aft, the pattern of every battleship for the next 80 years, and the first British warship to dispense with sail entirely.

She steamed out to Melbourne in 1871 and guarded Port Phillip for the Victorian Navy, the Commonwealth Naval Forces and the Royal Australian Navy until 1924 without ever firing a shot in anger. Sold for scrap, her hull was bought by Sandringham Council and on 2 September 1926 scuttled off Half Moon Bay as a breakwater for the Black Rock Yacht Club. For decades she was a dive and snorkel site with the turrets and breastwork standing above water.

Corrosion caught up with her: by the 1990s the hull was sagging, in 1993 the deck collapsed, and the wreck has now collapsed to the waterline, crushing the passageways in the lower hull. The four guns were lifted off in 2005 and placed on the seabed beside the hull, where they undergo in-situ conservation with sacrificial anodes. A 0.5-hectare protection zone surrounds the site and diving or snorkelling within it is prohibited without a Heritage Victoria permit.

What the visitor gets is a view: the rusting breastwork and turret bases 200 m off a popular beach, best appreciated from the cliff path or a kayak, with the story told on interpretive signs and by the Friends of the Cerberus. The reef along the Half Moon Bay cliffs outside the zone is a pleasant shallow snorkel with leatherjackets, sea stars, sponges and the occasional seahorse. Cerberus is included here for completeness and because divers regularly ask about it; the answer is that the world's only surviving monitor of her type is a heritage site, not a dive.

Half Moon Bay is at the end of Cerberus Way off Beach Road, Black Rock, about 18 km south of the Melbourne CBD. There is a paid foreshore car park above the beach and the Black Rock Yacht Club, with a short ramp down to the sand; the Cerberus sits in plain view 200 m offshore. Divers and snorkellers may use the beach and the sandstone reef along the cliffs to the north and south, but must stay outside the marked 0.5-hectare protection zone around the wreck – Heritage Victoria and the Cerberus preservation trust enforce it and penalties are severe.

The Black Rock life saving club, toilets and the Cerberus Beach House cafe are at the car park; fills at bayside dive shops.

Half Moon Bay faces west across Port Phillip and takes the full fetch of westerly and south-westerly winds, which build chop and stir the sandy bottom; easterlies and calm mornings give clear water on the reef. No ocean swell reaches here, but the bay's short steep chop in a 20-knot westerly is enough to make the shoreline unpleasant and the wreck is the yacht club's breakwater precisely because of it. Visibility on the surrounding reef is 2–6 m, best in autumn after dry weather.

None of this applies to the wreck itself, which cannot be entered.

Not diveable. Snorkellers can follow the sandstone reef north or south of the beach in 2–4 m staying outside the buoyed protection zone; the four guns removed from the wreck in 2005 lie on the seabed within the zone and cannot be visited.

No operator – diving and snorkelling within the protection zone is prohibited without a Heritage Victoria permit, granted only for research and conservation work. The Friends of the Cerberus conduct permitted inspection dives. The Half Moon Bay reef outside the zone is snorkelled independently.