Point Cooke Marine Sanctuary is the largest of Port Phillip's marine sanctuaries at 290 hectares, a broad sweep of shallow water off the Point Cook Coastal Park on the bay's north-western shore, and Point Cooke Beach is its shore entry. The geology is the same lava that built the Williamstown reefs: basalt flows that reached the bay and now form low cliffs and ledges rising out of a muddy seabed, with seagrass beds between the rock. Parks Victoria describes rocky reefs rising from shallow water offshore interspersed with seagrass, and that is the dive.

From the beach it is a long wade across intertidal sand and rock flats, as much as 200 m, before the basalt reef proper begins in 2 m and steps down to about 5 m. On the rock are tube worms, anemones, sponges, sea urchins and colourful algae; in the seagrass, pipefish, gobies and juvenile fish; and on the sand between, southern fiddler rays basking in the shallows, skates, small sharks including Port Jacksons and draughtboards, flathead and stingarees. Southern blennies live in every crevice and bottlenose dolphins pass along the sanctuary edge in some seasons.

Two heritage wrecks, the Henrietta and the Diana, lie within the sanctuary to the east. It is more a snorkel and a nature-study site than a dive: depth is slight, the wade is long and the water is often turbid because it is so shallow and so close to the Werribee River plume. But for divers in the western suburbs it is the only sanctuary reef on their side of the bay, the setting inside the coastal park with its picnic areas and toilets is pleasant, and a calm high-tide morning in autumn shows a surprisingly intact basalt-and-seagrass community.

Snakes are common in the park and blue-ringed octopus in the rock pools.

From the Princes Freeway take Point Cook Road south for 6 km through Point Cook Coastal Park to the Point Cooke Beach Recreation Area, or approach via Point Cook Homestead Road. There are several car parks off Side Entrance Road; the walk from the nearest to the water is about 250 m across the reserve and a narrow sandy beach. Enter from the beach and wade or fin across shallow intertidal sand and rock flats that extend up to 200 m offshore before the basalt reef rises and the water deepens to 2-5 m.

Exit the same way. The site is remote for a suburban dive; tell someone your plans. The heritage wrecks Henrietta and Diana lie 1.5 km east and are covered separately.

Best with light to moderate offshore westerly to north-easterly winds, or light onshore easterly to south-westerly winds. Avoid strong southerly winds, which put chop across the long shallow flats and stir the mud. Do not dive after rain, when Skeleton Creek and the Werribee River plume reduce visibility to nothing.

Diveable at any tide but high tide is ideal, both for depth over the flats and for the long wade. Visibility 2-5 m, best in winter. No swell.

Jellyfish swarms in late summer.

Enter from the beach at high tide, wade and fin across the flats to the reef edge, descend at 2 m and follow the basalt ledges seaward and along the shore at 3-5 m, keeping the reef between you and the beach for the return, turn at half gas or 25 minutes and come back across the flats.

No regular operator – dived independently; Parks Victoria licensed tour operators occasionally run snorkel tours. Boat access from Williamstown or Werribee South ramps.