At first, I didn't get the ending. I just couldn't buy the fact that the boy died. However, it does make sense. Since the only one jumping, after the boy went into the ocean was the mermaid. And nobody would take in the boy's imagination and see is as a good thing. Not even his parents.
First I thought "was it bad enough to die for?". But when the world is against you, and you lose all hope, I guess it is that bad.
I think this film represents another message which is "caring for each other". If anybody cared for the boy and praised his imagination, he wouldn't have to die. I think this film was strong and provoking.