People lined up from 8am to toss off their inhibitions and bathers to swim the 900 metres in Sydney's Middle Harbour.

''There was a 89 year old bloke, someone flew in from overseas, there were four paraplegics, a double amputee, there were fat people, thin people, young and old,'' said organiser Nigel Marsh.

''It was just joyous,'' said Mr Marsh. ''I am flouting on cloud nine. It is probably one of the nicest days of my life.''

One participant, who preferred that his name not be used, described the event as lovely, warm, sunny and relaxed.

"I don't think anyone was embarrassed," he said. "I think people just enjoyed the naturalness of the occasion.

Swimmers left their clothes on the beach before dashing to the water, where groups of 50 people at a time swam the 900 metres from Cobblers Beach near Sydney Harbour.

The swimmers were in varying states of buffness. They included a woman who wore only a bright pink floral cap and a mother with a toddler. Some people used flotation devices, like noodles.

The idea of a nude ocean swim was dreamt up by Mr Marsh, the author of Fit, Fifty and Fired-Up, who wanted to use it as an opportunity to throw aside concerns of body image, to connect with nature and celebrate as a community.

It was not an event for nudists, he told Fairfax Media, but a chance for people who wouldn't normally get naked to try something new.

But the swim did attract support from naturalists, including a group that calls itself "I Love Being Naked And Proud Of It," which posted updates this morning on social media saying, "I love nude swimming."

Mr Marsh said he had a "bit of a life change" more than 10 years ago when he did the Bondi to Bronte ocean swim. He used that event to determine "to be a better man and to be better husband".

While the final amount raised is still being calculated, Mr Marsh estimated the swim raised as much as $15,000 for the Foundation for National Parks & Wildlife.

''We will definitely be back next year,'' he said, but with thousands and thousands of people.