Yes, this film is based on the experiences of a Spanish family who were caught in the terrible tsunami that began on 26 December 2004.
How closely the film follows the experiences of this family is not sure, as they don't seem to have been publicly interviewed yet. The film depicts them as a family of five, with an older son and two younger children. The elder son and the mother get separated from the father and the younger ones and the film focuses on the family's efforts to survive and find each other again.
So whether it all happened exactly as depicted we won't know until we hear the comments from the family. However, the movie has been praised for its realism and for trying to show the effects of the tsunami on the whole region rather than just this one family. There have been some criticisms about making a movie based on one European family in this Asian disaster, though.