Yep, this is the one.
Raymond,
"What are you doin' now?"
Eugene,
"I'm sinking."
Raymond,
"Why you sinkin'?"
Eugene,
"I don't know."
Raymond,
"How come i'm not sinkin'?"
Eugene,
"Because your standing on regular sand and i'm standing on quicksand."
Raymond,
"There's no quicksand around here."
Eugene,
"Well then congradulate me because I just discovered some!"
LOL!

I haven't watched this movie in ages, but the way I remember it is Martin Short's character is sinking in the quicksand, looking all calm, and Danny Glover's character is freaking out asking him why he's so calm, to which he replies that he read if you struggle then you will sink faster. I just remember Martin Short calmly sinking, was quite funny.

Yeah this movie does have a part where he gets stuck in quicksand. It's really funny!