Marmion Marine Park, declared in 1987 as Western Australia's first marine park, stretches about 20 km from Trigg Island to Burns Beach and protects more than 136 fish species, ten seagrass species and 21 seabird species. Little Island is a small, flat sand and limestone island off Hillarys, inside a sanctuary zone. It is one of three places near Perth where the public regularly sees Australian sea lions hauled out, and DBCA's guide describes it, with Three Mile Reef, as an offshore reef with better visibility than the inshore sites.

The Marine Life Network describes a shallow, protected site of 5–10 m around the island, good for snorkelling, with seals and seabirds. The reef has ledges and low overhangs covered in algae, and supports buffalo bream, wrasse, sweep, herring cale, western blue devils and schools of baitfish, with crayfish, stingrays and cuttlefish. Bottlenose dolphins are common, humpback whales pass in spring, and turtles are occasionally seen.

Most sea lions near Perth are males that move between here, Burns Rock, Carnac and the Shoalwater islands and breed at colonies off Jurien Bay every 17–18 months. In the water they are curious and may come close. Keep at least 10 m away, never approach animals on the beach and do not feed or touch them.

Little Island suits snorkellers and divers of all levels on calm days, and it is one of the easiest wildlife encounters in Perth. Beaching kayaks is prohibited. Humpback whales pass the island from September to November, when whale-watching boats also work the area.

Launch at Hillarys Boat Harbour (public ramps off Southside Drive) or the Ocean Reef boat ramp further north. Little Island lies a short run north-west of Hillarys, within the park's offshore reef line. Anchor in sand well away from the island and the sea lions.

Kayakers paddle out from Hillarys or Sorrento, but beaching kayaks is not allowed and walking on the beach is discouraged. Allow 10–15 minutes by boat.

Marmion's reefs are exposed to south-westerly and westerly ocean swell and to the afternoon south-westerly sea breeze (the Fremantle Doctor), which in summer usually arrives between late morning and early afternoon. The island needs calm weather: swell under about 1 m and light winds. Visibility is generally better than at the inshore reefs.

North-westerly winter fronts make it unsafe.

Anchor in sand at a respectful distance, snorkel or dive over the shallow reef and ledges around the island in 3–8 m, and let sea lions approach if they choose. Stay out of the island's intertidal area.

Visited by Hillarys-based dive and snorkel charters (including Image Dive & Charters) and wildlife tours; private boats and kayaks.