The Jawbone is a lobe of the Newer Volcanics lava that reached Port Phillip at Williamstown, and its shoreline is a chaos of basalt boulders, ledges, seagrass and saltmarsh that has been protected since 2002 as the Jawbone Marine Sanctuary, backed on land by the Jawbone Flora and Fauna Reserve. It is not an easy access: the walk from the Esplanade parking is 250 m or more via the Bay Trail, the entry is over slippery rock and shingle, and there are no facilities. The reward is one of the more distinctive habitats in the bay.

The bottom is basalt boulders thickly encrusted with tubeworms and mussels, with sea stars, crabs, shrimps and common sea urchins in every gap, abalone on the rock, nudibranchs on the sponges and a good fish community of zebrafish, dusky morwong, leatherjackets, wrasse and blennies. Seasonally there are dumpling squid, seahorses in the seagrass, octopus in the boulders and small sharks (Port Jacksons and draughtboard sharks) resting under ledges. Depth is 2-6 m and the reef grades outward into seagrass and sand.

Because it is a no-take zone the abalone and crayfish that were stripped from the rest of the western shore are slowly returning, which makes it a useful comparison with the unprotected reef outside. It is calmer than Williamstown Beach breakwater in a southerly because East Point breaks the fetch, and it is a good, if effortful, night dive. Dive it at high tide; at low water it is snorkelling depth at best.

The Jawbone West entry and the sanctuary overview are documented separately.

From the West Gate Freeway take Melbourne Road then Kororoit Creek Road into Williamstown and turn onto the Esplanade; park roadside on the Esplanade between Bayview Street and Victoria Street, in front of Gloucester Reserve. The direct walk is 250 m or more: back into Bayview Street, along the Bay Trail bikeway and down to the entry just north of East Point. In the right conditions a 90 m walk straight across Gloucester Reserve to the shore is possible.

Enter over basalt boulders and shingle at high tide, taking care on the slippery rock. Exit at the same point. There are no toilets or changing facilities here.

Best with light to moderate offshore northerly to easterly winds, or light onshore south-easterly to north-westerly winds; strong southerlies and south-westerlies put chop onto the Williamstown shore and stir the silt. Dive at high tide, which is also when the intertidal reef is accessible to snorkellers. Avoid the site for two or three days after heavy rain, when Yarra and stormwater runoff cut visibility to a metre.

Visibility 2-6 m in good conditions, best in winter. No swell; the water is flat when the wind is still. More sheltered than Williamstown Beach breakwater because East Point breaks the fetch.

The intertidal reef is only accessible at high tide and the site is very shallow at low water.

Enter north of East Point at high tide, fin out over the basalt boulders, descend at 2-3 m and follow the reef edge west and seaward at 4-6 m, turn at half gas or 25 minutes, and return along the same edge to the entry.

No regular commercial operator – dived independently and by western-suburbs dive clubs and Melbourne shop social dives; no shop is based in Williamstown, so fills come from Dive Victoria Group or Scuba Culture.