Sienna is quoted by a British newspaper as saying: "There were eight of us and we were playing hippies and it was the summer of love.
"We take our clothes off, eight boys and girls, and go for a swim.
"We were nervous but it's real camaraderie when you're all together. It's actually hilarious getting naked in front of people you love and respect.
"The water was freezing cold, so the boys weren't too happy. It was colder than anything I've experienced in my life.
"But as a result it was quite funny and we felt a big sense of achievement after doing it."
'Hippie Hippie Shake' is adapted from the memoirs of Australian journalist Richard Neville, who launched the magazine Oz in London in the 60s and ended up on trial for printing sexually explicit material.
'28 Days Later' star Cillian Murphy takes the role of Neville, with Sienna playing his girlfriend Louise Ferrier.
'Hippie Hippie Shake' is due to hit cinemas next year.