Penguin Island is the centrepiece of Shoalwater Islands Marine Park and home to the largest colony of little penguins in Western Australia. It is a 12 ha limestone island a short ferry ride from Mersey Point, with boardwalks, a discovery centre and beaches that are open to visitors for most of the year. Underwater, the DBCA describes cavernous reefs that make good diving and snorkelling.

The reef is low limestone with small caves, overhangs and ledges, mostly in 2–5 m. The best areas are off the north and south ends of the island. On the southern beaches the Little Penguin Trail lets snorkellers follow the rock base from an entry in 2–3 m, over reef where schools of baitfish, the penguins' food, gather. Buffalo bream, wrasse, sweep, leatherjackets and herring cale are common, and divers see octopus, crayfish, rays and sea stars. Australian sea lions from nearby Seal Island and bottlenose dolphins often come by, and little penguins can occasionally be seen swimming.

This is a sheltered, shallow site that suits families, snorkellers and new divers. Visibility is variable and suffers after westerly swell. Boardwalks restrict access on land, and some beaches may be closed for wildlife.

The island's biggest hazard is on land and in the shallows between it and Mersey Point: the sandbar looks walkable, but currents and waves have swept people off it, and the council and DBCA warn against crossing it. Use the ferry or a boat.

Penguin Island is best combined with Seal Island and the Point Peron reefs for a day of snorkelling in Shoalwater Bay.

Drive to Mersey Point, Shoalwater (Arcadia Drive, off Safety Bay Road, about 10 minutes from Rockingham, 50 minutes from Perth). The Penguin Island ferry runs from the Mersey Point jetty daily from about 9 am to 3 pm between mid-September and early June; book ahead in school holidays. On the island, snorkel the Little Penguin Trail from the southern beaches, entering in 2–3 m.

Private boats and kayaks launch at Mersey Point or Point Peron. The sandbar between Mersey Point and the island is dangerous: people have drowned crossing it, so do not wade across.

The island shelters its eastern and southern shores from the prevailing south-westerly swell, but visibility is poor after swelly weather and westerly winds push surge into the reefs. Dive or snorkel in the morning with easterly winds and swell under 1 m; the summer sea breeze roughens the water by early afternoon. The island itself closes to visitors in winter (roughly June to mid-September) for the penguin breeding season, although boat-based dives around it are still possible.

Never cross the sandbar on foot.

Snorkellers follow the Little Penguin Trail around the rock base off the southern beaches in 2–5 m. Divers from boats anchor in sand off the north or south end of the island and explore the low limestone reef, small caves and overhangs at 3–5 m, returning to the anchor.

Penguin Island ferry and wildlife cruises from Mersey Point operate daily in season; Rockingham Wild Encounters-style snorkel cruises and local dive schools include the island reefs. Private boats and kayakers visit independently.