Half Moon Bay is the sheltered cove below the red sandstone bluff at Black Rock, best known for the hulk of HMVS Cerberus, the 1870 breastwork monitor scuttled as a breakwater in 1926, which sits 270 m offshore and is now so unstable that a protection zone bans entry. Divers still come to the bay for the small jetty and boat ramp in its southern corner. The jetty dive is shallow and easy: pylons in 2-4 m carrying mussels, sea squirts and sponges, with small leatherjackets, gobbleguts (southern cardinalfish), sea stars, anemones, urchins and nudibranchs, the occasional seahorse and, in the holes and under debris, southern blue-ringed octopus, which should be admired and left alone.

West of the jetty the bottom is open sand and seagrass at 5-6 m, dull by day but transformed at night, when octopus, flathead, bull rays and stingarees move over it and the plankton lights up with every fin stroke. Spring night dives here are a Melbourne club tradition for that reason. The site's other attraction is shelter: tucked under the bluff and facing west, it is diveable in most winds except southerlies, and the small size of the bay makes navigation trivial.

It suits new divers, night-dive courses, snorkellers and anyone with an hour to spare on the bayside. Parking is the practical limit; the car park at the bottom of the hill is small and metered and can be full by mid-morning on a fine weekend. Ricketts Point Marine Sanctuary begins a kilometre to the south-east and its Quiet Corner entry is the next site along.

From Beach Road at Black Rock turn down the steep access road into Half Moon Bay (signposted Black Rock Yacht Club and Half Moon Bay Life Saving Club). The metered car park at the bottom is small and fills quickly on fine days, so arrive early. The jetty and public boat ramp are right beside the car park, a carry of under 50 m.

Enter down the ramp (keep clear of launching boats), swim out under the jetty to its end and then head west across the sand flats at 5-6 m. Do not approach within the marked 0.5 ha protection zone around the Cerberus. Exit back up the ramp.

Best when the wind is from the north-west; good in anything except a southerly, which makes the bay choppy and stirs the sand. Dive just before high tide for the clearest water. Visibility 3-6 m typically, better in winter, reduced after rain.

No swell. Boat ramp traffic is constant in summer, so enter and exit to one side of the ramp and tow a flag. The bay is an excellent night dive, particularly in spring when bioluminescence is at its brightest.

Enter at the ramp, follow the jetty pylons out to the end (about 4 m), take a westerly heading across the sand and weed patches at 5-6 m, turn after 15-20 minutes or at half gas, return on a reciprocal to the jetty and ascend in the shallows beside the ramp. Stay outside the Cerberus exclusion zone to the north-west.

No regular commercial operator – dived independently and by Melbourne dive clubs and shop social-dive programs (Scuba Culture, Aquability, Melbourne Diving Services run occasional shore-dive nights at bayside piers); Ricketts Point-based snorkel tours run separately. Half Moon Bay is a common club night-dive venue in spring.