Astrolabe Cove – La Perouse Bay, or Larpa Bay to locals – is the shallow stretch of mainland shore between the Bare Island bridge and La Perouse Point, named for one of the two ships of the French expedition under Lapérouse that anchored here in January 1788, six days after the First Fleet. It is a modest but surprisingly rich dive that most people walk straight past on the way to the island. From the old boat ramp beside the bridge the bottom is sand at 3 m with a wall about a metre high; drifting north-west the reef edge runs along the shore and then west around the head of the bay, where the wall grows to four metres with small boulders and a few overhangs off it, and the depth settles at about 10 m along the edge.

It is a critter dive. Big-belly seahorses cling to the sponges and sea squirts, red Indianfish and pineapplefish (knightfish) hide under the overhangs, blue-ringed octopus live in the shallows, weedy seadragons appear along the sand, and at night squid gather in numbers. The tidal flow along the shore changes direction from section to section but is never strong, and the dive is best done as a slow drift on the incoming tide, exiting back onto the platform after about 40 minutes or continuing around La Perouse Point on the Larpa drift.

Visibility is the weak point – the head of the bay silts easily – and the local rock fishers are not always friendly. It suits divers who want an easy, sheltered macro dive or a night dive when Bare Island itself is crowded, and it connects the museum and monument precinct of La Perouse with the water in a way few Sydney dives do.

Park near the road down to the Bare Island bridge at La Perouse, preferably on the western side. Walk down the access road, cross onto the rock platform on the mainland side west of the bridge and enter near the old boat ramp in 3 m of water. Drift north-west along the reef edge and around the head of the bay; exit back onto the rock platform north-east of the wall after about 40 minutes, ascending the wall to 7 m first, or continue to the La Perouse Point slipway.

Sheltered inside the bay behind Bare Island and diveable in almost any wind, though north-easterly swell over 1.5 m creates surge on the shallow shelf. Tidal currents flow in opposite directions along different parts of the shore but are never strong enough to be a problem; the dive is best done as a gentle drift on the incoming tide, which also brings the best visibility. Visibility is often poor (5–10 m) because the bay silts up after rain.

Rock fishers on the platform can be hostile – McFadyen was hit by a thrown bottle here in 2005 – and their lines are a snag hazard.

Enter at the old boat ramp, follow the reef edge north-west through the shallow rocky section for about 20 minutes, turn west at the head of the bay along the taller wall with its gullies and boulders, continue south-west towards La Perouse Point and after about 40 minutes swing north-east back to the rock platform, ascending the wall to 7 m before the shallows.

Occasionally used by Abyss and Sydney Dive Academy for macro and night dives; mostly independent divers.