I'll confess right up front I'm a fan of the grim ending... however, those who are not can think of this as the last moments just before the young lady is rescued/rescues herself from the bog... I've also touched on aspects of this in other threads... but I'm trying to bring it all together.
For me, this is the pay off of any sinking scene... after all of the struggling, the mud/sand/peat/clay flying about, the moment of truth, as the victim slips out of sight in the heaving slime. She's now chin deep and going under. Her eyes dart about for help which isn't coming.... and she knows this is it.
Now, she either tips her head back, or stares straight ahead as the muck creeps over her mouth, her nose, and lastly her eyes and forehead. The top of her head goes next, and that's that. Her breath explodes on the surface in a spray of bubbles. A hand or two may pop up to wave goodbye, or claw helplessly at the surface, before it too vanishes. The camera lingers on the wobbling muck, maybe a few more bubbles appear.... and then all is still. End scene.
If she is whimpering or her last cries are muffled by the mud covering her mouth, that works well for me. Even better is if she is gurgling or sputtering, trying in vain to keep the muck out of her mouth and nose as she sinks under. I keep hoping for a mouth filling with quicksand, but I suppose for sanitary reasons that is being unreasonable. But, a man can dream....
Face plants - as in sinking face first or sideways - kind of spoils it for me. It isn't a game changer, but it does hurt things a bit. I mean, to me, someone going under would be trying to keep their face clear of the quicksand as long as possible. But maybe others like that.
Hands moving too fast also breaks the illusion for me as well. Ideal earth deathtraps should act as if they are thick and inescapable... fast moving hands kind of show how watery they can be.
Anyway, that's my two cents... anyone have any thoughts on this?
Not necessarily grim endings, but I always feel a tinge of disappointment when they don't go under, not matter what the scenario. As Bart said, one of my favorites is the girl going under, but then the hero comes along and pulls her out and then gives her mouth to mouth. The girl's chest heaves as she begins to breath again and then looks up into the eyes of her savior. 
That look of recognition to not only realize that she is alive when only seconds from death, but she would not be there if it were not for you/me. 
Of course then there is the clean up later where the girl must take off her clothes (if she's not nude already) in order to get clean. My favorite her taking a shower and inviting me to join her as a reward. 
