spoiler
In the In Water ending, there is a close-up of James looking defeated, starting the car, and gripping the steering wheel just before it fades to black.
In the Stillness ending, the last thing he does on camera is take a thoughtful look at Mary's letter (the one that tells him to move on with his life). He starts the car off-camera.
There's already a theory based on the audio in the Stillness ending that he gets out of the car and lets it drive itself into the lake with Mary's body. I feel like this difference in camera shots makes it a lot more obvious.
Anyway, 