Seal Island is a small limestone island in Shoalwater Bay, a short hop north of Penguin Island, where Australian sea lions haul out to rest. It is one of three places near Perth where the public can reliably see sea lions, and snorkel cruises from Rockingham come here to let visitors watch them in the water. The island itself is closed to landing.

Underwater, the island is ringed by shallow limestone reef, sand and seagrass in 1–6 m. The reef has small ledges and overhangs with buffalo bream, wrasse, sweep, herring cale, leatherjackets and seasonal baitfish schools, and the seagrass holds cuttlefish, rays and pipefish. The main attraction is the sea lions: mostly young and adult males that travel from breeding colonies further north. They are curious and playful in the water and may swim close to snorkellers. Bottlenose dolphins and little penguins from Penguin Island also use the bay.

Australian sea lions are an endangered species and unpredictable. Keep at least 10 m away, let them decide how close to come, never approach animals on the beach, and do not feed or touch them. Sea lions have bitten people. Operators follow DBCA wildlife interaction guidelines, and private boats should do the same.

The site suits snorkellers and divers of all levels who want a sheltered, shallow wildlife encounter. Visibility is usually moderate. Combining Seal Island with Penguin Island and the Point Peron reefs makes an easy half-day in Shoalwater Islands Marine Park, with short boat runs between each stop.

Launch at Mersey Point or the Point Peron boat ramps, or join a wildlife and snorkel cruise from Rockingham or Mersey Point. The island is a few minutes by boat north of Penguin Island in Shoalwater Bay. Kayakers use the Shoalwater paddle trail from Point Peron Road, which passes Seal Island.

Anchor in sand only, well away from the island, and do not land: the island is closed to the public to protect the sea lions.

Shoalwater Bay is sheltered by the island chain and Garden Island from most south-westerly ocean swell, but westerly winds and swell reduce visibility and make the reef edges surgy. Mornings with light easterly winds are best; the summer sea breeze roughens the bay from midday. Sea lions are present year-round, but numbers vary with their 17–18-month breeding cycle at the Abrolhos and Jurien colonies.

Anchor in sand at a respectful distance from the island, snorkel or dive over the surrounding shallow reef and seagrass in 2–5 m and let sea lions approach if they choose. Do not swim towards hauled-out animals or enter the island's intertidal area.

Wildlife and snorkel cruises from Rockingham and Mersey Point (seasonal, several times a week); local dive schools run occasional sea lion snorkels and dives. Otherwise private boats and kayaks.