This is the only thread you need to read... now read it (spoilers)
Everyones question: Is that Elly in the morgue at the end?
If the answers yes, it has to be that she killed herself by drowning? When the cops get there to investigate after they save the boy, they say any body will wash up on shore by morning. THIS IS IMPORTANT. Unless I'm wrong, at least a couple days go by before they end up at the morgue at the end of the movie. For that time to lapse, she had to be overcome with guilt and killed herself within a day or two of leaving the boy to die in the water. If she killed herself, she must of chose drowning to make it look like she tried to save the boy. Drowning isn't a fun dead to go through I'm guessing. I don't think she'd put herself through that just to make herself look better after death.
My guess is that it isn't her in the morgue. The blank look on the fiancés face is because he still knows he lost her forever. He knows that she bailed on everyone and he won't see her again... same as her being dead really. He doesn't tell the "friends" outside anything because they're all liars and they are partly guilty for what happened... they don't deserve to know anything, they're strangers for god sake.