Yanchep Lagoon is a 300 m stretch of sheltered water behind a limestone reef that runs north to south about 50 m off the beach at Yanchep. It is a popular family swimming and snorkelling spot with a surf club, cafe and new car park and showers close to the beach. For snorkellers the southern end, with its rock pools, is usually the best.

The long reef is divided into an inside (lagoon) section and an outside section, with smaller reefs 100–200 m to the south-west. Howies describes little channels and gullies and a low swim-through in a cracked piece of reef, with a maximum depth of about 5 m. Snorkelling guides report baitfish, butterflyfish, wrasse, crayfish and corals, and the reef supports the usual Sunset Coast species such as buffalo bream, sweep and herring cale.

The lagoon is protected from small swell, but the site is unforgiving in anything more. Howies found strong surge, mucky water and a dangerous undertow on exit, and warns that conditions need to be perfect and to have been so for days. Lifeguards also prohibit diving inside the lagoon while the flags are up.

Yanchep suits snorkellers and families on calm days and experienced shore divers who can judge conditions. For scuba, it is best treated as an opportunistic site for a glassy morning.

Water temperatures on this coast range from about 17 °C in late winter to 22–23 °C in late summer, when the warm Leeuwin Current is strongest, so a 5 mm wetsuit suits summer and a 7 mm suit with a hood is more comfortable from June to October.

Drive north on the Mitchell Freeway and Marmion Avenue to Yanchep (about 45–60 minutes from the Perth CBD) and follow Brazier Road to the new car park by the lagoon, which has showers close to the beach. The surf club, toilets and a cafe are nearby. Enter from the beach at the lagoon's sheltered section.

Lifeguards do not allow diving inside the lagoon when the patrol flags are up, so dive outside patrol hours.

Conditions must be perfect: swell under 1 m, a period around 11 seconds and preferably low easterly winds, sustained for days beforehand. Otherwise it gets murky and surgy, with powerful water movement over the reef and a strong undercurrent at the beach that makes exits exhausting. The southern end of the lagoon is calmest.

Dive early before the sea breeze and outside lifeguard patrol hours.

Enter at the sheltered section and explore the inside of the reef along its gullies and channels in 2–4 m. On very calm days experienced divers can visit the outer reef and smaller reefs 100–200 m south-west. Plan the exit carefully.

No operator; snorkellers and occasional independent divers.