Jawbone Marine Sanctuary protects 30 hectares of coastal water at Williamstown, tucked between the suburb's swimming beach and the mouth of Kororoit Creek, and it is the only place in the inner west where a full sequence of Port Phillip habitats survives together: intertidal basalt from a Newer Volcanics lava flow, saltmarsh and mangrove-fringed lagoon behind it, seagrass meadows, boulder reef and patches of small kelp in the deeper water. Parks Victoria describes seagrasses, boulders and local kelp forests, and that is what a diver sees: a shallow, sheltered, complex bottom that is the best snorkelling in the western suburbs and a good shallow dive at high tide. The basalt boulders carry tubeworms, mussels, sponges and algae and hide crabs, shrimps, abalone, urchins, sea stars and nudibranchs; the seagrass holds pipefish, seahorses, juvenile fish, cuttlefish eggs and dumpling squid; and the fish include zebrafish, dusky morwong, leatherjackets, wrasse, blennies and juvenile snapper, with rays, fiddler rays and small sharks visiting from the sand outside.

Because the sanctuary has been a no-take zone since 2002 it is a reference site for how the western shore looked before abalone and crayfish were fished out. The difficulties are access and depth: every entry means a walk along the Bay Trail and a scramble over slippery boulders, there are no facilities at all, and much of the sanctuary is under a metre of water at low tide, so this is a high-tide, calm-day, warm-season site more than a regular dive. The Scuba Doctor lists the sanctuary as a pointer to several entries, of which The Jawbone (East Point) is documented separately; this record covers the sanctuary overall and the western ground.

Park on the Esplanade, Williamstown, near Bayview Street or in the streets behind Gloucester Reserve; some visitors also approach from the Kororoit Creek Road end via the Jawbone Reserve paths, respecting the fenced bird-protection areas. All entries involve a walk of 100-300 m along the Bay Trail and a scramble over basalt boulders; the East Point entry (documented separately as The Jawbone, Williamstown) is the best defined. The western entries reach the seagrass and boulder ground on the sanctuary's outer edge.

Dive at high tide only. There are no toilets, showers or change rooms anywhere in the sanctuary.

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. Extremely shallow at low tide; plan the dive around the top of the tide.

Enter at high tide at East Point or a western gap in the boulders, fin out over the intertidal basalt, descend at 2 m and follow the boulder reef seaward into the seagrass and small kelp patches at 3-6 m, work along the outer edge, turn at half gas and return on a reciprocal 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. Parks Victoria and Hobsons Bay council volunteers run occasional guided snorkels.