As much as I love The Princess Bride, it perpetuates the myth that quicksand will take you under. And as Wesley so aptly stated, “We are men of action, lies do not become us.”
You may think it inconceivable, but research shows that quicksand doesn’t suck you in. It’s more dense then water and the greater the density, the greater the buoyancy. This makes objects take longer to sink. The conclusion of a MythBusters episode on this subject was that a victim would die from some other reason.
Here’s a video of a Brainiacs episode about quicksand that illustrates the same point.