The Titanic star was close to running out of air in his tank when he took off after a school of stingray and strayed from his diving party.
He tells the new issue of Rolling Stone magazine, "I was trying to get footage with a rinky-dink camera. I looked down, because I was exerting myself a lot, and I think the tanks weren't fastened properly, and I had, like 40 pounds of pressure."
Too deep to simply swim to the surface, DiCaprio took one deep breath and rejoined his diving group who shared oxygen with the star.
He admits there was a moment when his life flashed before him: "All you think about at that moment is, like, 'Get me the hell out of this situation'."