First Rock is a large limestone rock in the Shoalwater island chain with a reef around its base and a network of caves running beneath it. The City of Rockingham lists it as a scuba site in 2–5 m and rates it for divers because of the caves.

The rock and its reef are covered in kelp bombies and red algae. In the caves and under the overhangs live western wobbegongs, octopus and buffalo bream, and the reef supports the usual Shoalwater fish: wrasse, sweep, herring cale and leatherjackets. The sandy gutters around the rock hold rays and cuttlefish. As elsewhere in the marine park, Australian sea lions and dolphins can appear.

The caves are shallow and exposed to surge, and the council warns that winds and swell over 1 m make the site dangerous. That limits it to calm days. Even then, keep to open sections of the caves where you can see daylight, as the limestone is sharp and silt builds up inside.

First Rock suits Open Water divers with good buoyancy, and snorkellers on flat days, who want something with more structure than the open reefs. It is close enough to the ramps to combine with Penguin Island, Seal Island or the Sisters.

Published detail is limited to the council's guide, and GPS coordinates are not published, so this record should be checked against local knowledge before use. Ask Rockingham dive shops for the current position and conditions before heading out, and carry a torch to look into the caves from their entrances.

Launch at Mersey Point, Point Peron, Bent Street (Safety Bay) or Port Kennedy and run a short distance into the Shoalwater island chain; First Rock is one of the rocks south of Penguin Island within the marine park. Anchor in sand clear of the reef. Journey time from the ramps is around 10 minutes.

The City of Rockingham's diving guide numbers it as site 8.

The City of Rockingham warns that winds and swell over 1 m make conditions dangerous, because the cave networks are shallow and surge moves through them. Dive on calm mornings with light easterly winds, before the sea breeze. Visibility drops after westerly swell.

North-westerly winds ahead of winter fronts close the area, so the site is mainly a summer and autumn option.

Anchor in sand beside the rock, descend and circle the base in 3–5 m, looking into the cave networks that run beneath First Rock from outside. Enter only wide, open sections in calm conditions with good buoyancy.

Occasionally visited by Rockingham dive schools; mostly private boats.