Black Rock Shoal is a low sandstone reef sitting on the sand plain a kilometre or so off Black Rock, between Half Moon Bay and the Ricketts Point Marine Sanctuary. It is one of a handful of natural offshore reefs on the inner eastern shore and, being out of reach of shore divers, it sees far less traffic than the sanctuary reefs inshore. The shoal rises from about 10 m on the surrounding sand to within 3 m of the surface, with a broken top, short ledges and undercuts around its margin and patches of kelp and mixed brown algae on the crest.

The fish life is the same suite found at Ricketts Point but bigger and less shy: snapper, dusky morwong, magpie perch, zebrafish, old wives, leatherjackets, moonlighters and blue-throat wrasse, with Port Jackson sharks resting under the ledges in winter, smooth stingrays and eagle rays on the sand and the usual sea stars, urchins and nudibranchs on the rock. Cuttlefish are regular in spring. Because it is a small isolated structure it concentrates life, and because it is a little deeper than anything reachable from the beach it is a useful step-up dive for new divers from a club boat.

Documentation is thin; The Scuba Doctor lists the mark with coordinates and a 3-10 m range but no narrative, and the description here is built from that listing and the adjacent sanctuary reefs. Treat it as a pleasant, easy small-boat dive for a calm day rather than a destination.

Black Rock Shoal is a small-boat dive. Launch at the Half Moon Bay public ramp (limited parking) or from the Sandringham or Mordialloc ramps and run 1-3 km to the mark; it is a 5-10 minute trip from Half Moon Bay. Anchor on sand beside the reef rather than on it and fly an A-flag; the shoal is close to the Ricketts Point sanctuary boundary and to the main inshore boat lane.

Fit, experienced divers occasionally reach it as a long surface swim from Quiet Corner but it is not a sensible shore dive.

An open-bay reef with no shelter: diveable only in light winds, ideally northerly to easterly, and blown out by any fresh southerly, south-westerly or westerly that raises chop across the eastern shore. Visibility is best on the high tide and in the cooler months and is poor after rain. No ocean swell.

Small boats should watch for afternoon sea breezes, which build quickly in summer.

Anchor on sand on the lee side, descend the line to 6-8 m, circle the shoal keeping the rock on one shoulder, spend time on the deeper outer ledges at 8-10 m, and return to the anchor at half gas. No safety stop is required at these depths but a slow ascent up the line is good practice with boat traffic overhead.

No regular operator – private boats and club RIBs only.