Stealth And Detection In A Marsh
In a moor, the maximum distance at which a creature can succeed at a Perception check to detect the presence of others is 6d6×10 feet. In a swamp, this distance is 2d8×10 feet. Vegetation and deep bogs provide plentiful concealment (20% miss chance), so it is possible to use Stealth to hide in a marsh.
Source
Core Rulebook pg. 398