According to Naomi Watts, the producers of the film heard an interview on Spanish radio where the family told their story of surviving the Tsunami and decided to make a film based on it.
For the underwater scenes, Naomi Watts was strapped in a rotating chair. While filming, Watts couldn't hold her breath any longer and she gave the 'stop' sign. Instead of stopping, the chair started spinning the other way around. Later it was revealed that it was a technical error and Watts stated the she felt very scared and hated the chair for that reason.
This movie has been one of the latest produced at the so called "Ciudad de la Luz" studios (City of Light studios), in Spain. At October 2012 this complex announced its immediate closure due to the small number of productions that have taken place in the studios.
When Henry is first shown after the tsunami hits, his hair is dry and mostly neat. He immediately starts calling for Lucas and Maria, and his hair all of a sudden is very wet.
Old Woman: Some of those stars have been burnt out for a long, long time. They're dead, but once they were so bright that their light is still travelling through space. We can still see them.
Why were Maria and Lucas hit by a second wave coming from the opposite direction to the first wave?
A
It didn't come from the opposite direction. After they got hit by the first wave, the water changes direction and flows back into the ocean, meaning that the second wave was also coming from the ocean. This is unclear in the film, yes, but with a little common sense, this can be deduced. Think about how waves break normally on a beach: the wave comes in and washes on the shore and is pulled back out, what's called "undertow". The same principle applied in this scene only on a much larger scale.
Q
What's the long, stringy thing Maria pulls out of her mouth when she's vomiting?
A
Most likely it's a piece of seaweed. Though we don't see what happens, a piece of seaweed had somehow forced its way down her throat while she was struggling underwater. The force of the water rushing inland had probably filled Maria's throat faster than she could realize and a lot of water was forced into her stomach with the seaweed.