Meet the site
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.
Research this site before booking Stored site notes may be historical. These questions highlight gaps without turning old instructions into a current dive plan.
Cow Rocks (Marmion) is recorded as Reef in Perth. The listed access is boat. Confirm the actual meeting point, route and permission to visit; a map pin does not establish any of these.
Topics flagged by the stored research
These are questions prompted by the existing notes, not a verified or complete hazard assessment.
- Other water users
- Ask about vessel activity, exclusion areas and the agreed surface arrangements.
- Lines and obstructions
- Discuss any known entanglement concerns and the suitability of the planned area.
- Water movement
- Ask how swell and local exposure affect the proposed entry, exit and route.
Historical access notes exist for this record, but have not been independently verified for current use. Request current directions from the operator or site manager rather than following an old entry description.
Questions, answered
Is Cow Rocks (Marmion) suitable for my certification?
The record lists Open Water. That does not establish suitability for every route or day. Share your training, recent experience and equipment needs with the operator and ask about the specific proposed dive.
Can I book Cow Rocks (Marmion) through YouDive?
YouDive is an independent research directory. Contact an operator directly to confirm whether it currently runs the trip, the inclusions and the booking terms.
What you might see
Blue groper, western rock lobster, Port Jackson sharks, buffalo bream, wrasse, scalyfin, western blue devils, old wives, morwong, sponges, corals
Location
Inshore rocks and reef in Marmion Marine Park near Hillarys, listed by DBCA as a boat-access dive site.
Facilities nearby
Hillarys Boat Harbour ramps and services.
Before you make plans
Conditions, access and operator schedules can change. This is a location guide, not a dive plan. Ask a local operator about current conditions and suitability for your training and experience.
Sources & further reading
exploreparks.dbca.wa.gov.autrailswa.com.audivessi.comRecord updated 26 Sept 2026. Source claims are being reviewed.
