In the film, the Charlie's Angels star plays Rachel Kramer, an environmental activist who launches a worldwide campaign to free a family of whales trapped in the Arctic Circle - and for one scene she had to plunge into the ocean.
Barrymore admits her attempts at authenticity underwater were instantly met with excruciating pain.
During an appearance on U.S. breakfast show Good Morning America on Wednesday (25Jan12), she explained, "The director of this film, Ken Kwapis, he had us in Alaska on this ice flow, we felt like we were working in a snow globe theatre... I got to swim in freezing cold Alaskan waters with harbor seals...
"It felt like knives going into my body. It was insane. They told me the temperature was 34 degrees (Fahrenheit) and I was like, 'That doesn't sound so bad.' Then you get in and you're like, 'Ouch.' I'm very bad at pretending or faking things, I really do want to transport myself and sort of believe in whatever scenario I'm in."
But Barrymore insists the experience wasn't all bad - she was fortunate enough to meet the heroes behind the real-life 1988 rescue operation: "We worked with people and families who have been in Alaska for generations, whose parents were involved in the rescuing of the whales... It was great."