The British beauty, 18, suffered acutely from seasickness and found the pills she took to combat her illness had an unwanted side-effect.
Keira explains, "Normally I don't get seasick at all. But I'd never been on an 18th-century boat before and it rocks - a lot. So when we were 20 miles out I did decide that a seasickness tablet was a good idea.
"But they make you very drowsy - beyond drowsy. There are outtakes where I literally fell asleep mid-take."
But Keira and the rest of the cast - including Hollywood star Johnny Depp - had a more frightening encounter when their speedboat began to sink when it was returning them from a day's filming.
She says, "The boat began going down and we had to jump out. Then we realised we'd washed up on a reef. So we were wading through water up to our chests and we waited there in the dark for an hour until two Frenchmen came and rescued us.
"It was one of the most surreal experiences of my life."21/07/2003 17:07