The specifications for getting each ending (if I remember correctly) are:
Leave:
-Keep your health up for most of the game.
-Don't do the other stuff to get the other endings.
In Water:
-Have low health through most of the game.
-Examine Angela's knife a lot.
-Examine some journal/newspaper on a roof that talks about...something.
Maria:
-Don't let Maria take a lot of damage.
-Don't listen to the headphones recording.
-In the hallway where you hear Mary's narration, go to the end of it and exit the room before she finishes talking.
There's some other stuff I'm missing, but yeah. In Water is the most common ending.
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Black Turtle did a pretty good job.