The 40-year-old Heathridge man was diving with a friend about 1km off shore when trouble struck about 7.30am.
Whitfords Volunteer Sea Rescue Group Commander Roger Howell said his team heard a distress call and arrived about 10 minutes later to find the victim not breathing.
The diving partner, who had set off a flare, had been working with a helper from a nearby boat in attempts to resuscitate the man.
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READ NOWAn ambulance crew was on hand to treat the man at Mindarie Marina, but could not revive him.
Initial reports suggested an equipment malfunction may have led to the emergency, but Water Police Sergeant Paul Crawshaw yesterday said it was unlikely.
‘Equipment malfunctions that have previously been reported may have been done so hastily,’ he said.
Police are still investigating.
Commander Howell said the men had followed the correct safety procedures while diving.
‘It appeared it was nothing silly, nothing that could be put down to foolishness or anything like that,’ he said.
‘It was just one of those awful things.’
He said fatal diving incidents were uncommon, with the Whitfords crew having attended ‘no more than a dozen fatalities in the last 20 years’.