Dan and Amy didnt make it
Here's why
I am surprised that no one in the long thread about the ending seems to have noticed that when Amy dives there was a sea storm. Do you really think that a woman (or a man) would survive that ? even if she was not far from the boat. (nature is way stronger than a man or woman)
plus bringing a man heavier than her and dead? and don't tell me that Dan was still alive we see him clearly dead going to the bottom, and earlier she said that she almost couldn't feel her legs (water becoming colder with time)
She was a ghost , but i don't think Dan was there lying , it was her spirit mind that put him there to remind her she took the wrong decision between saving Dan or stay with her baby and that forever , like her own hell, condemned to wander on the ship forever.
The baby was crying and turned his arms on the right and the left trying to reach someone but there was no one to reach to . no one was there and the shots made it clear there was no one on the deck, there was even an aerial shot of the boat and suddenly the screen goes white (limbo) and suddenly we see Amy and Dan on the deck (like i said Dan isn't really there reason above)
Some have said that she must be a blind and deaf ghost cause she didn't notice there was another boat around but there's plenty of ghost stories (real or not) when the ghost doesn't seem to notice anyone or anything around
Plus this film is based on a novel by a Japanese writer and guess what , yeah it's a ghost story . Amen