Boat Cave is exactly what the name says: a sea cave in the north-west cliffs big enough for a small boat to shelter in, and the operators sometimes moor the dive boat inside it. Extra Divers describe it as equal parts reef and adventure, with a depth range of 4–16 m and a torchlit finale in the cave.

The cave has an above-water section extending back into the cliff, and a floor at about 8–10 m that is completely covered in plate corals, angled towards the entrance to catch the light (Diveplanit, Divernet). The walls and ceiling carry red, pink and yellow whip corals and other colourful invertebrates, which come alive in torchlight. It is shallow and roomy enough that most divers feel comfortable, and the light from the entrance is always visible.

Outside, the reef along the base of the cliff has healthy hard corals and plenty of reef fish, and the blue beyond brings pelagics: Extra Divers mention wahoo and turtles, and the north-west corner of the island is known for passing sharks, dolphins and, in the wet season, whale sharks. Most dives spend the first part on the reef and finish in the cave.

Boat Cave is an easy introduction to Christmas Island's sea caves, suited to divers with basic buoyancy skills who want a taste of the island's karst without committing to Thundercliff or Thunderdome. It is on the more exposed north-west part of the coast, near Perpendicular Wall, and needs calm conditions. Photographers enjoy the plate-coral floor framed by the cave mouth.

Fly from Perth (about 4 hours, two to three flights a week, some via Cocos Islands) and hire a car – there is no public transport. Both dive centres are in The Settlement and launch their boats from the jetty/boat ramp at Flying Fish Cove, normally at about 8 am for a two-dive morning trip. From the cove the boat runs west and south-west along the north coast along the coast for roughly about 30–35 minutes to Boat Cave, between West White Beach and Perpendicular Wall.

There is no land access to the dive, and the vertical sea cliffs mean no exit other than the boat. Operators choose sites on the morning according to swell and wind.

On the exposed west coast, a long run from Flying Fish Cove, so it is only dived on calm days. The site is sheltered from the south-east trades (roughly May–November) but wide open to north-westerly monsoon swell (December–April) and to long-period south-west groundswell generated in the Southern Ocean, which wraps around Egeria Point. Operators generally need less than about 1–1.5 m of swell and light winds to make the trip; the calmest spell is usually August–November.

The west coast has a reputation for stronger and less predictable currents than the north, and for the clearest water on the island (often 40 m+). Visibility can drop during red crab spawning (Oct–Jan). Cyclone season is November–April.

Boat entry into the cave is possible only in flat calm.

Start on the reef outside at 10–16 m, working along the cliff, then finish by swimming into the cave at 8–10 m over the plate-coral floor, torches on to light the whips and fans, and surface or do the safety stop in the entrance.

Extra Divers Christmas Island – regular dive, sometimes with the boat moored inside the cave; CI Wet'n'Dry Adventures – occasional.