Green Island is a small island off the south coast of Rottnest near Nancy Cove, and one of the island's five marine sanctuary zones. The Rottnest Island Authority says the Green Island Sanctuary Zone protects critical habitat for male Australian sea lions, an endangered species that hauls out on the island. SeaLink's snorkel guide describes the area as a thriving marine ecosystem where you may see sea lions, and recommends it for more adventurous snorkellers in suitable winds.

The reef around the island is limestone with gutters, ledges and seagrass typical of the south coast, and the combination of sanctuary protection and clear southern water supports good fish life. The chance of a sea lion encounter in the water is the main draw, but these are wild, protected animals: keep your distance, do not approach animals on the rocks, and let them choose whether to interact.

No dive-specific description was found, so depths given here are indicative. Green Island suits confident snorkellers and shore divers prepared to cycle a long way with gear, or boat divers exploring the south coast on a calm day.

Combine a visit with Salmon Bay and Little Salmon Bay on the ride back toward the settlement, choosing whichever bay is in the lee. Bring drinking water, as there is none at the beach, and plenty of sun protection for the ride.

Cycle or take the Island Explorer bus west along the south coast road toward Nancy Cove (roughly 12-14 km from the settlement; allow 45 minutes or more by bike). Leave bikes above the beach and enter from the sand to snorkel toward Green Island. Boats can come from Fremantle (about 75 minutes) but must observe sanctuary rules and keep clear of sea lions.

SeaLink lists suitable winds as north, north-east, south-west, west and north-west. Exposed to southerly swell; dive only on calm days. The south coast gets rough quickly when the swell rises, and afternoon sea breezes chop up the surface.

General pattern at Rottnest: summer brings calm morning easterlies followed by a strong south-westerly sea breeze from late morning, so dive early; winter brings north-westerly fronts and large swell, with clear, calm spells between them. Check the Bureau of Meteorology coastal forecast for swell height and period before committing.

Snorkel from Nancy Cove toward the reef around Green Island, staying well clear of any sea lions on the island, explore the reef edges and gutters, and return to the same beach. Agree a turn pressure before descending (half gas, or a third where current is likely), carry a DSMB, and make a 3-minute safety stop at 5 m.

Self-guided snorkel; guided snorkel tours occasionally visit. No scuba operator schedule found. Nearest dive services: Perth Scuba, Bucket List Diver and Dolphin Scuba (Perth/Fremantle) run Rottnest boat trips; there are no dive shops or air fills on the island.