Carnac Island is a small limestone island and A-class nature reserve about 10 km off Fremantle. It is best known for its Australian sea lions, which haul out on its beaches, for its seabird colonies and for a large population of tiger snakes, which David Attenborough filmed for a documentary. For boaters, the sheltered eastern bay is a popular day anchorage for swimming and snorkelling.

For divers, the Marine Life Network describes the best reefs to the south and south-west: a weed-covered limestone reef about a mile wide with small caves that hold plenty of marine life. The Dark Cave area on the west side, about 14 m deep, has many sections of reef with caves and crevices. Schools of juvenile hammerhead sharks have been recorded in summer. Reef life includes buffalo bream, wrasse, sweep, herring cale and western rock lobster, and bottlenose dolphins are common.

The sea lions are the highlight for many visitors. They are mostly males resting between breeding seasons and may investigate snorkellers in the water. Keep at least 10 m away, never approach animals on the beach, and let them choose how close to come.

Visibility can be poor, and the outer reefs are exposed, so choose calm days. Landing is limited to the beach, the interior is closed and full of tiger snakes, and closures are applied at times for wildlife.

Carnac suits snorkellers and Open Water divers on calm days, and it makes an easy half-day trip from Fremantle. The run back to Fremantle can be rough once the afternoon sea breeze arrives, so plan to leave by early afternoon.

Launch at Woodman Point, Fremantle Fishing Boat Harbour or the Fremantle Sailing Club; Carnac is a short powered-boat run (15–20 minutes) or a 45-minute sail from Fremantle. The sheltered bay on the eastern side is the usual anchorage for snorkelling. For diving, the best reefs are to the south and south-west, and the Dark Cave area is on the west side in about 14 m.

Anchor only in sand, and check DBCA notices for seasonal closures before landing.

The eastern bay is sheltered from the prevailing south-westerly swell and wind and is the calm-weather snorkel spot. The southern and western reefs are exposed and need swell under about 1 m. Visibility is sometimes poor, especially after westerly weather.

Dive in the morning before the summer sea breeze; winter north-westerlies make the eastern bay uncomfortable.

From an anchorage off the southern reefs, descend to 8–12 m and explore the weed-covered limestone and its small caves, keeping away from the sea lion haul-out beaches. On the west side, the Dark Cave area has reef sections with caves and crevices to about 14 m.

Wildlife and snorkel tours from Fremantle visit several times a week in season; dive schools and clubs make occasional trips; many private boats.