Throughout the film, Curtis' dreams have a specific narrative and style. They are all strictly from his perspective and never include what other people in the dreams are perceiving. When Curtis and his family are at Myrtle Beach, it cuts away from him and shows that his daughter sees something, then it cuts to his wife who is no where near Curtis coming out of the house. It shows a far away shot of Curtis, which is clearly in her perspective, then it shows her noticing the oil dripping on her hand, also from her perspective. Also, each dream has a dream-like feel to it, the people's faces who attack Curtis are obscured, his wife blankly stares at him and moves very slowly; if the sequence at the end is a dream, then it is by far the most realistic one.
It's implied that the ending scene is indeed reality, confirming that Curtis is not having hallucinations or delusions, and that he was in fact having premonitions. However, in the commentary that takes place with the DVD, Michael Shannon and the Director refuse to comment on the meaning of the last scene at Myrtle Beach. They speak in detail about the construction of the scene but refuse to give comment on the meaning, and state that they never will: that it is all up to the interpretation of the viewer. Seeing that Curtis is finally getting real treatment for his problems, and that he has his family's love and support of this, some view this last scene as metaphoric. He's having a dream, but others are experiencing the same frightening observations that he is. Curtis and his wife look understandingly at each other (something the director and Shannon acknowledge as being the most important part of the scene) and there is no panic, there is no violence. This may be an acknowledgement that Curtis is not alone, he has support, and he is on a road to some type of recovery.