Marmion Marine Park, declared in 1987 as Western Australia's first marine park, stretches about 20 km from Trigg Island to Burns Beach and protects more than 136 fish species, ten seagrass species and 21 seabird species. Cow Rocks is one of the named inshore reefs in DBCA's list of Marmion boat-access dive sites, and SSI's Perth guide names Cow Rock alongside Boyinaboat Reef and North Lump as a feature of the park, with blue groper, crayfish, Port Jackson sharks and corals.

The park guide describes the inshore reefs as having intricate overhangs, ledges, caves and swim-throughs. Cow Rocks follows this pattern: limestone rising from sand and seagrass, kelp-covered on top and undercut around the edges. Divers can expect buffalo bream, wrasse, scalyfin, western blue devils, old wives and morwong, with lobsters and Port Jackson sharks in the ledges and sponges and small corals on the walls.

No detailed published guide to Cow Rocks was found, so depth and position are not given here. Local operators and the sounder are the best guides.

The reef suits Open Water divers wanting an easy boat dive close to Hillarys. It is a good alternative to Boyinaboat for divers who prefer a boat entry to the breakwater scramble.

The inshore reefs are within easy reach of the harbour, and dives are usually timed for early morning to finish before the sea breeze. Carry a surface marker buoy, as boat traffic is heavy on weekends. Water is coolest, at about 17 °C, in late winter and early spring.

Launch at Hillarys Boat Harbour (public ramps off Southside Drive) or the Ocean Reef boat ramp further north. Cow Rocks are among the inshore reefs close to Hillarys; DBCA lists them as requiring boat access. The run is short, about 5–10 minutes.

Anchor in sand beside the reef, and check sanctuary zone boundaries first. Hillarys Boat Harbour has parking, toilets, fuel and dive shops, and it is about 25 minutes from the Perth CBD via the Mitchell Freeway and Hepburn Avenue.

Marmion's reefs are exposed to south-westerly and westerly ocean swell and to the afternoon south-westerly sea breeze (the Fremantle Doctor), which in summer usually arrives between late morning and early afternoon. Dive in swell under about 1.5 m with light easterly winds and before the sea breeze. Visibility is excellent in summer when swell is low and falls after winter storms.

Anchor in sand beside the rocks, circle the reef following its ledges and overhangs, and return to the anchor.

Occasionally included on Marmion boat dives from Hillarys; private boats.