From the front, the antipodean actress looked fully covered-up with long flowing cape-like sleeves.
However, she was happy to flash the flesh with her dress slashed to the waist at the back.
The actress beamed on the red carpet last night for a film which she described as the "most physically demanding thing I've ever done".
The 44-year-old actress stars with Ewan McGregor and rising star Tom Holland in the true story of Maria Belon and her family, who were holidaying in Thailand in 2004 when the Indian Ocean tsunami struck.
The stars joined Ms Belon at a charity screening at the BFI Imax cinema in London, held in aid of Indian Ocean Disaster Relief, which helps families affected by the disaster.
Watts, a mother of two, praised Ms Belon's determination to stay alive and save her sons.
She said: "That was the thing that struck me -- how courageous and heroic she was."
The Impossible is due out in cinemas on January 1.