Castle Rock is a single large bommie standing on the limestone and sand floor of Bass Strait south-west of Point Lonsdale, outside the Heads. The Scuba Doctor listing describes it as rising 'majestically out of a limestone and sandy bottom to around 12.0 metres on top', with 'many undercuts, splits and ledges' in its flanks and 'jagged rock turrets on top' that give it its name. It is roughly 200 m in circumference and the listing notes that divers can circle it once or twice in a dive – the whole site is a walk around a castle.

Because it stands in open water it carries a different community from the walls inside the Heads. The turrets and upper flanks are covered in yellow zoanthids and encrusting sponges, but the sand around the base is weedy seadragon country, and the listing records them here along with nudibranchs and sea stars. The fish list is long and typical of the Victorian open coast: southern blue devils in the undercuts, old wives, longsnout boarfish, dusky morwong, sixspine leatherjackets, scalyfin, zebrafish, bluethroat and senator wrasse and schools of sweep. Southern rock lobster are found under the ledges.

The depth range of 12–20 m and the ebb-tide preference make this one of the few Heads-area sites genuinely suited to Open Water divers, and operators use it as an 'outside' dive for mixed groups on calm days, often pairing it with Lonsdale Arches or Lonsdale Bommies on the same reef. The limiting factor is swell rather than current: it lies fully in Bass Strait and is unworkable in a south-westerly sea.

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 20 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 20 minutes to the site.

The site is outside the Heads south-west of Point Lonsdale, reached by running out through The Rip on a calm day; from Queenscliff it is about 20 minutes, from Portsea 25. 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 on the ebb slack or a gentle ebb only, when the outgoing tide leaves this side of the reef in relatively still water. 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. It lies fully in Bass Strait, so any south-westerly swell over about 1.5 m rules it out; the best days are light northerlies in late summer and autumn.

Anchor or shot beside the bommie on the ebb. Descend to the top at 12 m, drop to the sand at 18–20 m and circle the rock – about 200 m round – with the wall on one shoulder, looking into the undercuts and splits for crayfish and blue devils; two laps are possible on a 40-minute dive. Ascend the rock, safety stop on the turrets at 5–6 m below the top or under a DSMB.

Dive Victoria Group (Portsea and Queenscliff) – double boat dives to the Heads walls most days of the week, site chosen on the morning by slack-water time and swell; Bayplay Adventure Tours (Portsea) – weekend and holiday charters; Red Boats (Queenscliff) – weekend Heads charters; Melbourne clubs (VSAG, Power House Underwater Club, Scuba Diving Melbourne) run their own boats here regularly.