Prison Island is a tiny sandy cay on the eastern side of the Cocos lagoon, visited by lagoon tours for its history and its nursery reefs. The name dates from the early settlement era of the 19th century, and local guides tell several versions of how it came about; Cocos Blue Charters' half-day trips include the island as a historical site. The official tourism snorkelling feature describes the cay as recently cleared of coconut palms and surrounded by a juvenile fish nursery, and the official snorkelling guide groups it with the Broccoli Patches as hidden gems of the eastern atoll reached with a local skipper.

Snorkelling is in shallow water around the cay, over coral patches, sand and seagrass where juvenile reef fish shelter before moving out to the reef; small blacktip reef sharks, turtles, parrotfish and wrasse are common. Offshore, the Prison Gardens dive site uses the deeper reef nearby. The snorkel is easy, suits families, and is often combined on the same trip with Horsburgh Island and the Phaeton wreck, giving a sampler of the lagoon's history and marine life.

Because it is out on the eastern side, it is best visited on calm days. Stories of the island's history vary, and visitors should hear them from the local guides. Tour groups are small, and the skippers know the lagoon's sandbanks and channels intimately – important on the eastern side, where shallow flats and coral heads make navigation tricky for anyone unfamiliar with the lagoon.

Bring a hat, reef shoes and plenty of water: the cay has no shade to speak of, and the reflection off white sand and water intensifies the sun.

Prison Island is on the eastern side of the lagoon, south of Direction Island, and is reached only by boat. Book a half-day lagoon trip from West Island with Cocos Blue Charters (glass-bottom boat, snorkel and history tour visiting Prison and Horsburgh Islands) or Cocosday Tours (motorised canoe safaris to the eastern atoll); the official guide notes the eastern-atoll snorkel spots are reached with a local skipper. The boat beaches or anchors off the cay and snorkellers enter from the shallows.

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.

The eastern lagoon faces the south-east trades, so trips to Prison Island are rougher and less frequent in the windy season.

Enter from the shallows beside the cay and snorkel slowly around its fringing reef, looking for juvenile fish among the coral and seagrass. Stay close to the boat and the island.

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.