Film School Rejects have an interesting interview with The Impossible director J.A. Bayona. Read the excerpt mentioning Naomi below, and find the full thing at the source linked below:
In terms of the technical side of the film, directors always say, “Don’t shoot on water and don’t work with kids.”
[Laughs] And doing that in English was even worse, because it’s not my first language. It was dealing with kids, water, a different country, great actors, and in another language…
Was shooting the tsunami as difficult as you thought it would be?
Yeah, especially for the actors. For us, it was a technical challenge, but it was physical for the actors. Naomi [Watts] spent a month in the water, and she was already having stomach problems on her third day of shooting. You can imagine how physically demanding it would be.