The snake can be found in virtually all locations, including Guarma. Higher concentrations tend to be along the eastern regions, especially Bluewater Marsh in Bayou Nwa.
The Fer-de-Lance snake is not exclusive to the remote island of Guarma. It can be found on the main map in the Stillwater Creek area just southwest of Thieves Landing. A known spawn point is on the land directly east of the pool of water where you find the Legendary Redfin Pickerel.