The 'Make Me' hitmaker was enjoying the beautiful sea in Hawaii when she found herself unable to breathe after being dragged under huge "six-foot waves".
Recalling the scary event, she said: "I went out and I thought it was like a wave pool, and I was like, 'This is so nice and refreshing and just beautiful.'
"But the thing I didn't think about is when you come back, the waves really come in hard. I got sucked under for like five minutes. I almost drowned. And I'm like, 'Where is my security? They're just going to let me die here.' And then I came in again and another wave took me under."
In her interview with Scott Mills for his BBC Radio 1 show, the 34-year-old singer also opened up about the music industry, admitting there are a lot of "divas" in the business.
Speaking in an interview, which will air on Wednesday (24.08.16), she added: "A lot of the people in the entertainment business, if you try to work with them or set up projects, a lot of times people just don't show up. A lot of people are divas in this industry. It's hard."
Meanwhile, Britney - who has Sean, 10, and Jayden, nine, with her ex-husband Kevin Federline - previously revealed her new album 'Glory' is "very chill".
She shared: "It's very exciting. I've been working on this project for a year and a half now, almost two years so it really means a lot to me. It's my baby. It's a little melodic, a little more not so poppy.
"It's kind of chill. Very chill. I'm old school. I like classic rock. I like the Rolling Stones. The Lumineers are really cool. Sade is nice. I like 80s music, too."