A giant inflatable duck is missing off the coast of Western Australia after strong winds whisked it into the Indian Ocean.

"Daphne" the duck was meant to be the mascot for last Sunday's Coogee Jetty to Jetty Swim.

President of the Cockburn Masters Swimming Club, Peter Marr, said he was setting up ahead of the event when Daphne took flight in strong winds.

Mr Marr said his attempts to swim after the inflatable were futile.

"I probably overestimated my ability and underestimated Daphne's hunger for freedom and she just bolted," Mr Marr told ABC Radio Breakfast.

"After 50 metres she was just gone baby, gone. I couldn't keep up."

Mr Marr said Daphne — who cost the club $900 and is the size of a small caravan — had been a point of contention in the club for the past year.

"I didn't want the duck whatsoever," he said.

"We're a small organisation trying to run a swim that makes money for our club.

"I knew as I was swimming I'd be accused of doing it deliberately."

Fisherman have reported sightings of Daphne, including a group who saw her 15 kilometres west of Rottnest Island soon after she broke free.

There was also a rumour she has made it as far as Geraldton, almost 400km north of Coogee Beach.

Mr Marr said the club was offering a reward for Daphne's safe return — swimming merchandise and free entry into next year's swim.

"We're expecting a ransom note," he said.

"We'd look after them if they looked after us."