Boullanger Island is a low limestone island of about 35 ha lying directly opposite Jurien Bay township. It is part of the Boullanger, Whitlock, Favourite, Tern and Osprey Islands Nature Reserve – home to the vulnerable Boullanger Island dunnart, a tiny marsupial found nowhere else – and its surrounding waters form one of the sanctuary zones of the Jurien Bay Marine Park, established in 2003.

The Turquoise Coast's inshore reefs are known for caves, grottos and overhangs covered in multicoloured sponges and anemones, and for an unusual mix of temperate and warm-water fish carried south by the Leeuwin Current. Around Boullanger the lagoon holds limestone ledges and small caves, patches of seagrass that are nursery habitat for western rock lobster, and fish such as buffalo bream, western king wrasse, harlequin fish, sweep, herring and baldchin groper. Because the island is a sanctuary zone, fish are more abundant and less wary than on fished reefs. Australian sea lions and dolphins are frequently seen.

The diving and snorkelling are shallow and relaxed, suitable for beginners and families. Divers with their own boats can explore independently; others can join Jurien Bay Oceanic's reef snorkel tours or dive charters. The Lubra wreck lies between Boullanger and Favourite Islands, making a short additional snorkel.

Kayakers can paddle across from the town beach on calm mornings, but should return before the sea breeze.

Jurien Bay is about 220 km (2.5 hours) north of Perth via the Indian Ocean Drive. Launch at the Jurien Bay Boat Harbour; Boullanger Island is about 3 km offshore opposite the township (5-10 minutes). Anchor on sand in the lee (usually the eastern side) and avoid anchoring on reef or seagrass.

The island is a nature reserve – landing is permitted on day-use beaches only; check DBCA signs. Jurien Bay Oceanic's reef snorkel tours and dive charters visit the lagoon reefs.

The Turquoise Coast is protected by a line of offshore limestone reefs and islands that break the prevailing south-westerly swell, creating a lagoon of calm, clear water inshore. The main enemy is wind: the strong southerly sea breeze ('Fremantle Doctor') blows most summer afternoons from about 11 am, and winter north-westerly fronts bring storms and poor visibility. Mornings from October to April are best.

Boullanger Island sits inside the protective reef line, so it is sheltered from ocean swell, but exposed to the afternoon southerly sea breeze, which makes anchoring and snorkelling choppy.

Anchor in sand in the lee, snorkel or dive along the limestone ledges and overhangs at 3-8 m, look into the grottos for crayfish and baldchin groper, then return over seagrass.

Jurien Bay Oceanic Experience (Jurien Bay Boat Harbour; est. 1996) – daily sea lion snorkel tours, reef snorkel tours, PADI courses and dive charters – reef snorkel tours and dive charters in the lagoon.