Lasseter's Reef is a small, isolated reef off Second Valley beach that, like the gold reef it is named after, can be hard to find. The Federation describes a 50 m diameter dome-shaped reef with varied marine life and resident leafy seadragons, while Wikivoyage describes a low limestone outcrop, roughly circular, rising from a sand bottom at about 8 m to a top at about 3 m, with a higher-profile structure on the offshore side, surrounded by seagrass. The reef carries heavy growth of mixed kelp and other seaweeds, and Wikivoyage notes it can be slightly silty, so visibility reduces locally when disturbed.

The seagrass surrounding it is prime seadragon habitat: the Marine Life Network lists leafy seadragons in nearby weed beds, along with blue devils, eels, whiting, drummer and tommy ruff. Wrasse, magpie perch and leatherjackets are common on the reef itself. Getting there is the challenge.

The swim from the beach is about 300 m and the Marine Life Network describes access as strenuous, requiring fitness. Using the telephone-pole-and-toilet-block transit helps, as does a GPS mark if arriving by boat. The site is very exposed and not safe in a big swell.

Once there, the reef is compact enough to explore thoroughly in one dive, and the combination of kelp-covered dome and seagrass edges offers a real chance of leafy seadragons away from the crowds at Rapid Bay. It suits fit Open Water divers with good navigation skills and patient photographers. Leave enough air for the swim back, or surface-swim the return with a snorkel.

Second Valley is about 1 hour 20 minutes (92 km) south of Adelaide via Main South Road; turn right at the Second Valley sign and follow the road down to the beach. Park at the end of the access road (limited spaces; busy on summer weekends); there are public toilets and a changing room but no drinking water. From the centre of the beach, swim about 300 m north (or 200 m north-east of the jetty).

The reef is tricky to find: the Marine Life Network suggests lining up the telephone pole with the left-hand corner of the toilet block as a transit. The swim is strenuous in full gear, so a surface marker and good fitness are needed. Boats can reach it from Wirrina Cove (Marina St Vincent) or Sunset Cove.

Second Valley faces north-west into the gulf. It is sheltered from southerly and south-easterly weather and best in easterlies, but it is very exposed to northerly through westerly winds and swell and is not diveable in a big swell. Summer and autumn mornings, before the south-westerly sea breeze, are most reliable.

Tidal currents are small inside the bay but can be felt off the headland. Visibility is often good in calm weather and drops after westerlies and rain. Lasseter's is very exposed and not safe in a big swell.

Swim out on the surface using the transit, descend on the reef and circle it at 5-8 m, then spiral up to the top at 2-3 m. Search the surrounding seagrass edges for leafy seadragons before swimming back to the beach.

Club and shop shore dives from Adelaide (Underwater Sports, Diving Adelaide) occasionally; mostly independent divers.