The incident happened about 10am on Saturday at Port Beach in the suburb of North Fremantle, police say.
Two teenagers on a boat saw what they believed was a shark attack and called emergency services.
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".
He said the teenagers "made a fantastic effort" and "reacted really, really well" by alerting emergency services and providing information to assist the search.
All beaches in the area have been closed.
© AAP 2021
Image:Peter Burka/Flickr