The scene where the tsunami hits the resort could only be filmed once. It would have cost too much money to rebuild the set.
Many of the extras were actual survivors of the tsunami.
The family on which this story is based were present during the whole shooting process. They were giving tips, especially María Belón who is portrayed by Naomi Watts. The whole family also attended the film's premiere at the Toronto Film Festival in 2012.
The crew filmed partially on location at the actual resort, since rebuilt, where the Belón family was holidaying when the tsunami hit. For the destruction scenes, a scaled model of the resort was used.
It took a year to put together the ten-minute sequence, a recreation of the moment the wave came crashing down on the luxury hotel in Khao Lak.