Castle Rock Beach is a pocket of sand between sandstone boulders on the Clontarf side of the Dobroyd peninsula, reached by a steep bush track off the Spit to Manly walk and overlooked by the little white Grotto Point lighthouse, which has guided ships through the Heads since 1911. The VIZ club mentions it as an alternative entry for the southern approach to the Dobroyd Head reef, and it is the only practical shore access on the Middle Harbour side of the head. The bottom is boulders and kelp fringing the cove, giving way to sand at 8–10 m, with the rock ledges and gutters of Grotto Point to the east and the long Clontarf shoreline to the west.

Marine life is that of the North Harbour reserve edge and Middle Harbour mouth: blue groper, luderick, bream, old wives and leatherjackets around the boulders, Port Jackson sharks and the odd wobbegong in winter, cuttlefish, octopus and morays in the crevices, stingarees and flathead on the sand, seahorses and pipefish in the seagrass of the cove, and, because the point sits in the tidal stream, more passing kingfish, salmon and tailor than the sheltered beaches see. The lighthouse point itself has deeper gutters and better sponge cover but is swept by the tide. There is no published dive guide; depths and layout here are indicative and the site is included because it is dived by locals and is the logical Middle Harbour-side access to the Dobroyd reef.

Its challenges are the steep track, the tidal stream at the point and constant boat traffic through the Middle Harbour entrance; its reward is solitude in a beautiful setting a few kilometres from the Spit. It suits experienced Open Water divers and snorkellers who read tides, and it is not a beginner's site.

From Sandy Bay Road, Clontarf, or the end of Ogilvy Road, Balgowlah Heights, take the Spit to Manly walking track to the signposted Castle Rock Beach turn-off; a steep track of about 300 m with steps drops to the sand (limited street parking at the top; no facilities). Enter from the beach and follow the rocks east toward Grotto Point and its lighthouse, or west toward Clontarf. The VIZ club names Castle Rock Beach as the alternative entry for the southern (Grotto Point) approach to the Dobroyd Head reef.

Exit back at the beach and climb out.

Deep inside the harbour these sites see no ocean swell; only a strong southerly or south-easterly wind raises a chop, and even then the dive usually goes ahead. What matters is water quality and tide. Dive on the incoming tide or at high water, when the flood brings cleaner ocean water up the harbour and the shallow margins have enough depth; low tide exposes rock and shortens the dive.

Visibility is normally 3–7 m and can reach 10 m on a winter flood; after heavy rain the harbour runs brown for several days and visibility drops to a metre. Boat traffic is heavy on summer weekends. Night dives are the best dives.

Grotto Point stands at the mouth of Middle Harbour where the tidal stream is strongest of any site in this group – the whole of Middle Harbour drains past it – so dive the slack at high water and expect a pull mid-tide. Boat traffic in and out of Middle Harbour and the Spit passes within metres of the point. The cove itself is sheltered from swell.

Enter at the beach, descend over sand at 3–4 m and follow the boulders east toward Grotto Point, staying tight to the rocks, down to the sand line at 8–10 m; turn well before the point if the tide is running and return along the same rocks to the cove. Surface only in the cove.

No operator; independent divers, snorkellers and the VIZ club.