The 'spectator free' event kicked off on Sunday at the secluded beach at Middle Head at the Sydney Harbour National Park.
The skinny dippers don't strip off until they are on the sand and are handed out sarongs to cover up with as soon as they exit the water.
Beachley, who is an ambassador for the fundraising event, was pictured in the water on Sunday.
The 'naked ocean swim that's not about nudity' offers 300 or 900 metre courses and is not a race, nor is it about being seen nude, organisers say.
Australian neurosurgeon Charlie Teo, World Champion surfer Layne Beachley and Author and Founder Nigel Marsh joined a record 1,335 swimmers from around Australia.