Middle Head is the wedge of national-park bushland and colonial fortifications that divides Middle Harbour from the main harbour, and its two small nudist beaches, Cobblers on the north and Obelisk on the south, give shore divers access to a boulder reef that few people have seen. The VIZ dive club's GPS surveys are the only detailed guide. From Cobblers Beach the sand slopes to a sand line at 7–8 m; about 210 m offshore lies a small wreck (an old launch or workboat) that is the usual objective, and east of it the coast is large boulder formations with crevices and swim-throughs running round toward the head.

From Obelisk Beach the sand line at 8–10 m runs east for some 450 m along the north-east segment of the peninsula, and this is where the sponge gardens are: shaded boulder faces with sponges, ascidians and soft growth comparable, at a smaller scale, to North Head's. The Beehive Casemate mine tunnel lies along this southern face. The fish and animals recorded by the club are varied for a harbour site: juvenile and adult Port Jackson sharks and crested horn sharks under the boulders, blue groper, schools of yellowtail, red morwong, crocodile fish, hammer octopus and other octopus, cuttlefish, eels and lobsters in the crevices, and sea turtles more than once.

Night dives are productive for the nocturnal species. Visibility is typically 4–6 m; 8–10 m is 'not common'. The cost is the walk: 450 m and a 37 m climb back to the car with wet gear, which keeps the site quiet.

The eastern tip abuts the ship channel and boat traffic is heavy at the north-eastern corner, so divers stay on the sand line close to the rocks and carry a float. Middle Head suits fit, self-sufficient Open Water divers and scooter divers who want unvisited harbour reef, and night divers who do not mind the climb in the dark.

From Mosman take Middle Head Road to its end in Sydney Harbour National Park (NPWS pay-and-display car parks near the old barracks). For Cobblers Beach, take the signposted track from the car park: 450 m with a 37 m descent, a moderate incline described by the VIZ club as 'nothing particularly hard' but a long way with tanks. For Obelisk Beach, a shorter but steeper stepped path drops the same height from Chowder Bay Road.

Both are nude beaches. Enter from the sand; at Cobblers the wreck lies about 210 m offshore and an 8–10 minute swim, at Obelisk follow the sand line east. Exit at the same beach and climb back up.

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.

Middle Head projects into the harbour entrance and its eastern tip lies against the main ship channel; the Obelisk side takes surge from a big southerly swell through the Heads, and the north-eastern corner is where boat traffic is heaviest. Tidal streams round the head can be noticeable on a spring tide. Carry a float; night dives reduce the boat risk.

Cobblers: enter from the beach, swim on a bearing to the small wreck at 8–10 m about 210 m out (do not overshoot into the channel), then follow the coast east around the boulders and crevices toward the head and back. Obelisk: enter from the beach and follow the sand line east at 8–10 m for about 450 m along the sponge gardens and swim-throughs of the north-east segment, then return shallower over the boulders. Scooter divers can link the two beaches around the head in about 1 h 40.

No operator runs it; the VIZ dive club has mapped both entries and scooter-circumnavigated the head; independent divers and occasional Dive 2000 or Plunge club dives.