Chowder Bay, at Clifton Gardens on the Mosman shore, is the muck dive by which every other Sydney critter site is measured. The bay is a pocket of calm water under the sandstone of Georges Head, with a Victorian-era public wharf, a netted swimming enclosure, a grassy reserve and, along its eastern side, the heritage naval buildings of the old submarine miners' depot where Plunge Diving now operates. Below the surface it is unremarkable to look at: sand and silt sloping from 2 m at the beach to 9–10 m off the end of the wharf, timber piles, the enclosure net, concrete blocks, mooring chains, an Admiralty anchor, a sunken yacht and launch, a Land Rover, purpose-built 'seahorse hotels' installed by the Sydney Institute of Marine Science, and a great deal of harbour rubbish.

All of it is habitat. The animal list is why people come from interstate. White's seahorses live on the net and hotels and are the flagship of a state-listed recovery program; striped, yellow and red anglerfish sit on the piles; tiger pipefish, pygmy pipehorses and seahorses hide in the weed; decorator crabs cover every rope; gloomy octopus, cuttlefish and squid hunt at night; moray eels, stingarees, numbrays, flathead, goatfish, pygmy and fan-bellied leatherjackets, blennies, gobies, threefins and, in autumn, juvenile tropical fish complete it, with nudibranchs by the dozen for those who look.

Diveplanit sums it up as a 'classic inshore harbour muck dive' where one day you see everything and another 'you might not see your own hand'. Visibility is 3–7 m and silt lifts with any careless fin, which is the only skill the site demands; it is otherwise as easy a dive as exists, and its car park empties of charges after 8 pm, so it is Sydney's night-dive capital. Fishing lines from the wharf are a real hazard – Michael McFadyen was hooked twice in one dive.

Chowder Bay suits beginners, macro photographers, night divers and anyone who has never seen a seahorse; it is dived and guided daily and is the best single reason to own a macro lens in Sydney.

From Military Road, Mosman, follow Thompson Street then David Street to Morella Road, which ends at the Clifton Gardens reserve (23 Morella Road is the operators' meeting point). Large Mosman Council car park: paid, about $8/hour to a $28 daily maximum on weekends, free after 8 pm, which is one reason night dives are so popular. Gear up in the car park, walk 100 m across the grass and out along the wharf on the left of the swimming enclosure, and giant-stride from the steps at the end; or use the sand slope to the right of the wharf.

The alternative access from Chowder Bay Road/HMAS Penguin through Georges Head to the naval wharf is by arrangement only (Plunge Diving's base). Exit at the wharf steps or the beach.

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.

Chowder Bay faces east toward the Heads and a very large southerly swell can produce a little surge on the wharf piles, but it is essentially an all-weather site; Dive Centre Manly calls it 'diveable in most conditions'. Fishing from the wharf is heavy and lines hang among the piles.

Descend at the end of the wharf in 3–4 m and work the piles and the outside of the swimming net slowly for seahorses, anglerfish and decorator crabs; then head east across the sand and silt at 6–9 m to the mooring buoy, seahorse hotels, concrete blocks, anchors and the small yacht and launch wrecks, looking for pipefish, pygmy pipehorses and octopus; return north-west to the wharf on a reciprocal and finish along the net. Stay off the bottom – a single fin kick clouds the site. Sixty to ninety minutes is normal.

Plunge Diving – based at Chowder Bay, guided dives, courses and night dives daily; Dive Centre Manly – regular guided dives and club dives ('15 minutes from the store'); Pro Dive Manly, Frog Dive, Dive 2000 (Neutral Bay) and Abyss – courses, guided macro and night dives most weeks; Sydney Dive Charters shore option. The most-guided muck dive in Sydney.