The Adamson Sponge Gardens are named for the Catherine Adamson, a 900-ton Aberdeen clipper that was driven onto the inner side of North Head in a southerly gale on the night of 23–24 October 1857 while waiting for a pilot, only two months after the Dunbar was lost on South Head. Twenty-one people died, and the two disasters together prompted the building of the Hornby lighthouse. Her remains lie scattered on the reef below the cliff, protected under the historic shipwreck legislation, and the reef around them carries what Sydney Dive Charters calls 'the best sponge gardens located on North Head'.

The site is on the same inner face as Old Man's Hat and the Waterfall and shares their structure: boulders and short walls sloping from the cliff to a sand edge at 23–24 m, with the richest growth on the vertical and overhanging faces between 15 and 22 m. Here the sponges, gorgonians, sea whips and soft corals are at their densest, in the same 60-plus species counted across the reserve, and the shaded undersides of the blocks hold basket stars, nudibranchs, decorator crabs and small cowries. Among the fish and larger animals are weedy sea dragons on the sand line, wobbegongs and crested horn sharks under the ledges, bull rays, blue groper, yellowtail and old wives, and in winter Port Jackson sharks and the occasional bronze whaler passing along the edge.

Pieces of the clipper – iron knees, ballast stone, concreted fittings – can be recognised by those who look, but they are not the point of the dive. The gardens face west into the harbour and are sheltered from the north-easter, diveable on most days, with a tidal current that suits a gentle drift. Visibility is 10–20 m with a thermocline at 15 m in summer.

It suits Open Water divers upward, photographers, and anyone interested in the human history of the Heads; it is part of the five-site North Harbour Aquatic Reserve circuit with Old Man's Hat, Red Indian Point, the Waterfall and Fairfax Lookout.

Boat only. Dive Centre Manly loads at Manly Cove beach beside the wharf; Pro Dive Manly and Sydney Dive Charters load at the Little Manly boat ramp (end of Stuart Street, Manly – free parking, set-up benches, toilets at the reserve); Sydney Dive Charters also picks up at Rose Bay public wharf. From Manly the run is 10–15 minutes across North Harbour to the inside of North Head, 20–25 minutes from Rose Bay.

Private boats launch at Little Manly, Roseville Bridge (Middle Harbour, 30 minutes) or Rose Bay. Boats anchor on the reef in 15–20 m or run in toward the cliff and drop the anchor when the bottom appears; there are no moorings, and many abandoned anchors litter the reef.

The inner face of North Head is the most sheltered ocean-quality diving in Sydney. It faces west-south-west into the harbour, so it is protected from the north-east sea breeze and from northerly and easterly swell, and is at its best when a strong north-easter is blowing and every ocean site is rough. It is exposed to southerly swell rolling through the Heads: a big southerly (over 2 m) sends surge along the cliff and dirties the water, though the deeper sponge gardens are usually still diveable.

Tidal current runs along the wall, east on the flood and west on the ebb, strong enough to plan a drift; slack water is easiest for novices. Visibility is 10–15 m on average, 25 m on winter westerlies, and drops after heavy rain when the harbour flushes brown. Ferry, shipping and small-boat traffic through the Heads is constant – ascend on the anchor line or a DSMB close to the cliff.

Descend to 18 m and follow the reef with the tide along the 20–24 m sand edge, keeping an eye out for scattered iron and ballast from the 1857 wreck among the boulders, then return shallower through the sponge-covered faces. Nothing may be touched or removed. Safety stop on the anchor line.

Sydney Dive Charters – regular guided double dives in the North Harbour Aquatic Reserve, naming Adamson Sponge Gardens as one of its five reserve sites; Dive Centre Manly and Pro Dive Manly dive the same stretch under the general 'North Head' label.