Question

if one gets caught in swamp he is advised to lie flat

Answer 1

Because if the area is more then force is less. So he doesn't sinks in.

Answer 2

Wen one is caught in a swamp, he is advised to low flat, as pressure decreases wen the area in contact increases, coz area is inversely proportional to the pressure. Thus, the person wont sink in deep in the swamp, n can be rescued by someone