Little Armstrong Bay is a quiet north-coast bay within the Armstrong Bay Sanctuary Zone, one of Rottnest's five marine sanctuary zones, which the Rottnest Island Authority says protects important seagrass meadows. The Authority lists it among its six best snorkel spots, describing gently lapping waves and gradually increasing depths suitable for children and beginners through to experienced swimmers, and SeaLink calls it a sheltered bay with calm waters and easy access, with colourful and curious fish in a maze of seagrass.

The seagrass is the point of the site. The meadows are nursery grounds for juvenile fish and home to pipefish, small cuttlefish and invertebrates, while the limestone reef patches between them host wrasse and bream. Because the bay is in a sanctuary zone, fish are unfished and tend to be less wary.

It suits families and anyone who wants a calm snorkel away from the busier Basin and Parakeet Bay, and it makes a pleasant, shallow shore dive for scuba divers who want to look for small creatures in the seagrass. Take care not to stand on or drag fins through the seagrass, which is the reason for the sanctuary.

The bay sits between Parakeet Bay and Geordie Bay to the east and Catherine Bay and City of York Bay to the west, so a cycle along the north coast can take in several snorkel stops in one day.

Rottnest's north-coast bays are the island's summer snorkelling grounds because they sit in the lee of the prevailing south-westerly sea breeze. The Rottnest Island Reserve bans spearguns and handspears across all island waters, so fish throughout the bays are relatively approachable, and the Island Explorer bus and flat cycling road link them all. The general rule is simple: in a southerly, snorkel the north side.

About 4 km from the settlement - a 13-minute ride west along the north-coast road, or Island Explorer bus Stop 17. Leave bikes at the top of the beach and walk down to the sand; the water deepens gradually from the shore. Snorkel gear can be hired in the settlement; scuba divers must bring filled cylinders from Perth on the ferry, as there are no fills on the island.

SeaLink lists north-east, east, south-east, south and south-west winds as suitable - it is sheltered from the prevailing sea breeze. North-westerly winds and winter storms bring swell into the bay. 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.

Enter from the beach, snorkel over the seagrass maze and reef patches as the water deepens, and return along the shore. Snorkellers should stay within easy reach of the exit point and swim with a buddy; scuba divers at these depths need no safety stop but should ascend slowly and surface near the float.

No operator needed - self-guided snorkel/shore dive. Snorkel gear hire from Pedal & Flipper and other outlets in the Thomson Bay settlement; guided snorkel tours (Rottnest Snorkel) run from Thomson Bay. Scuba divers bring cylinders from Perth - no fills on the island.