Horseshoe Reef is the second reef of the Port Noarlunga marine reserve, lying off Christies Beach a few hundred metres north of Port Noarlunga Reef. It is a small limestone reef whose shape gives it its name, with depths of only 1-6 m. The inside of the arc forms a calm pocket that is easy to snorkel, especially at low tide, while the outside edge drops to sand and seagrass.

The reserve boundaries were extended in 1993 to include it, and it is now within the Port Noarlunga Reef Sanctuary Zone of Encounter Marine Park. The reef is covered with brown algae and kelp, with sponges and soft corals in shaded crevices. Magpie perch, sweep, zebrafish, leatherjackets, silver drummer and old wives are common, and whiting feed over the surrounding sand.

Nudibranchs are found on the rock. In early summer, Christies Beach becomes part of the Port Jackson shark mating aggregation that centres on Port Noarlunga: Diving Adelaide notes adults on a ledge a few hundred metres north of the jetty and in the shallows at the southern end of Christies Beach, and egg cases can be found in the reef. The Marine Life Network considers Horseshoe Reef suitable for supervised novice divers, and snorkel guides rate it medium difficulty only because of the swim.

For divers, it is a relaxed shallow dive and a quieter alternative to Port Noarlunga when the jetty is busy; for snorkellers and families, it is one of the better reef snorkels in Adelaide's south. Bring a surface marker buoy for the swim, as small boats and jet skis launch from nearby ramps, and watch for sea nettles (jellyfish) in warmer months.

Christies Beach is 35 minutes south of Adelaide via the Southern Expressway. Park on The Esplanade near the Christies Beach Surf Life Saving Club (free parking). Enter from the beach and swim about 200-400 m offshore on the surface to the reef – a confident swimmer's distance in full gear, so use a surface marker and pick a calm, low-tide day to shorten the swim.

Boats can launch at O'Sullivan Beach, a few minutes north, and anchor on sand outside the reef.

Best with light winds, easterlies or a gentle northerly and at low tide, which reduces the swim and the current. Westerly and south-westerly winds and swell create surf on the beach and stir up the water; the summer sea breeze usually builds after midday. Visibility can be excellent in early summer on calm days, but drops after rain when the Onkaparinga River discharges to the south.

Swim out on the surface to the reef, descend on the outside edge and follow the arc of the horseshoe at 3-5 m, then finish inside the sheltered arc before swimming back to the beach.

Diving Adelaide runs Port Jackson shark snorkel and dive tours along Christies Beach in season (Nov-Jan); otherwise dived independently and by clubs; no scheduled boat trips.