Corrected entry: Throughout most of the film the male characters wear leather underwear beneath their sheaths, for censorship purposes. When in actuality there would be no such thing. As the sheath was merely a coverup for the male privates in primitive cultures.
Corrected entry: Dar was not struggling enough in the quicksand to sink that far down. Most people will only sink to chest level when they are as calm as he was.
Correction: Firstly, the density of the quicksand would depend on how quickly or far someone would sink. Struggling makes it quicker because it changes the air voids and the density. Also, you answered yourself when you said "most people". This could certainly be one of those situations when Dar is not "most people."
Wouldn't the fact that Dar fell into the quicksand, as opposed to stepping into it, make a difference? I always wondered why he didn't go under upon landing. It seemed pretty watery compared to videos I've seen online about wilderness survival.
Correction: The movie is set in a fantasy world. We have no idea what their traditions are concerning clothing.