The key points we must keep in mind :
- The Wikipedia article on the Open Water Adrift says one adult surives
- The baby is still crying even after the aging fisherman is no longer around the boat
- And the surviving adult can't be Lauren
The director of the movie indicates one adult survives and it cannot be Lauren because if it truly was Lauren she must have ordered a rescue team to search for the yacht.
But instead, nothing except an aging fisherman who appears to know nothing about the yacht comes along.
The reason Amy looks disappointed at the end is she risked her life once more to save Dan, but it was futile. Dan passed away and that is why he is with his face down. Nobody likes to see a corpse's face
We can still hear the baby crying at the end, after the fisherman's boat is gone.
If Dan and Amy truly died, the aging fisherman must have brought the baby onto his own boat after concluding that nobody else was on the boat.
What happened -
Amy brings unconscious Dan back into the boat.
Amy makes numerous attempts to bring Dan back to his consciousness.
Amy's attempts are futile, and Dan dies.
It has been a very stressful and exhausting night.
Amy finally falls asleep.
In the morning, the fisherman comes by.
He hears the baby crying.
He enters the ship.
He sees the baby crying, and Amy (sleeping) and Dan (no longer alive)
The fisherman concludes it is safe to simply leave the ship.
The biggest clue -
If the fisherman truly have concluded that there was nobody else except the baby on the yacht, the fisherman would have brought the baby into his own boat.
But we can still hear the baby crying after the fisherman goes away.
If the fisherman concluded that there was nobody else on the ship he would have brought the baby into his own ship. But he just goes away
Another hour passes. Amy wakes up. The baby is still crying. The sound of the baby crying was very likely what woke Amy up. Amy concludes Dan died and she brings Dan's body onto the surface of the yacht and puts him on the surface of the yacht with his face down. The baby keeps on crying, and Amy is disappointed that Dan did not survive, and closes her eyes. If that truly was the ghost of Amy then the ghost of Dan would be awake as well. But no, Dan is unconscious (and no longer alive) at the end of the movie which means Dan is gone but Amy isn't