Horsburgh Island is the uninhabited island at the north-western corner of the southern atoll, about 1 km² of coconut palms and sand no more than 2 m above sea level, named after the hydrographer James Horsburgh who charted the islands in the early 1800s. Its main attraction ashore is historical: during the Second World War the Ceylon Garrison Artillery installed two six-inch guns at the southern point for coastal defence, and they remain in place today, listed on the Commonwealth Heritage List. Lagoon tours stop here to see the guns and snorkel the shallow reef along the shore, and Cocos Blue Charters includes Horsburgh on its half-day history and snorkelling trips.

Underwater, the lagoon side has shallow coral patches over sand with reef fish, small blacktip reef sharks and turtles, while the island's outer reefs host some of Cocos' best dive sites – Cologne Gardens and Lynnies Leap to the west, Garden of Eden to the north, and Cabbage Patch and Rose Wall to the south-east near the Western Boat Passage. The Phaeton wreck lies about a nautical mile south-east of the island in Port Refuge, and the Catalina JX435 site between Horsburgh and Direction Island, so a boat trip can combine several heritage sites. Snorkelling here is best treated as part of a tour rather than a destination in its own right; public documentation of specific snorkel spots around the island is limited.

The passage beside the island has tidal flow and boat traffic, so snorkellers should stay on the lagoon side close to the shore.

Horsburgh Island is reached only by boat, a short run north from West Island across the western lagoon. Cocos Blue Charters' half-day trips visit Horsburgh and Prison Islands for history and snorkelling, and private skippers can drop visitors on the beach. Cocos Dive's north-western dive sites – Cologne Gardens, Lynnies Leap, Eden, Cabbage Patch and Rose Wall – are on the reefs around it.

Lagoon snorkelling is possible year-round, but the south-east trades (April/May–October, often 15–25 knots) chop the lagoon, stir up sand and make swimming harder; the doldrum season (November–April) usually brings glassy conditions and the clearest water. Tides matter more than swell inside the lagoon: shallow flats can become too shallow at low water, and tidal streams run through channels between islands. Heavy tropical showers are brief and have little effect on visibility because there is no river run-off.

Horsburgh sits beside the Western Boat Passage, where tidal streams run; snorkel on the lagoon side and near slack water.

Land on the lagoon-side beach, visit the guns, then snorkel the shallow coral along the lagoon shore, staying clear of the passage where tidal currents run.

Visited by local lagoon-tour operators: Cocos Blue Charters (half-day glass-bottom boat, snorkel and fishing trips, visiting Prison and Horsburgh Islands), Cocosday Tours (motorised canoe safaris to Pulu Maraya and the eastern atoll) and Cocos Dive on its day trips.