Emergency services continue to search for the 57-year-old man after two teenagers on a boat saw what they believed was a shark attack at Port Beach in North Fremantle about 10am on Saturday morning.
It is feared multiple sharks attacked the man, who is known as a regular swimmer at the beach.
WA Police Acting Inspector Troy Douglas said police were unable to confirm how many sharks were involved in the attack but said it was "obviously at least one".
Insp Douglas said the teenagers "made a fantastic effort" and "reacted really, really well" by alerting emergency services and providing information to assist the search.
It's also believed that the teens got everyone else out of the water following the attack.
All beaches in the area have been closed and will remain closed for at least 24 hours.
Fisheries Minister Don Punch said police have deployed a number of vessels to the area.
"I'd like to extend our heartfelt thoughts to the family of this man at this very difficult time," he said.
"This is a terribly sad and distressing situation."
Warnings have been issued via the SharkSmart website and Surf Life Saving WA's Twitter feed.
Beach users are advised to monitor SharkSmart.
APP with The Project.