"A water snake glided smoothly up the pool, twisting its periscope head from side to side"
Soldiers using periscope binoculars
Creative Commons Attribution Share AlikeSoldiers using periscope binoculars - Credit: German Federal Archive on Wikimedia Commons

The term "water snake" may refer to any snake that lives in or around fresh water, although some species are specifically named "water snake" (eg in the genus Enhydris). They are usually quite small and coloured to match the dappled reflections in water.

A periscope is a device used for seeing things from a concealed position; on a submarine, it shows what is above the surface of the water. As much of a swimming snake's body may be underwater, its protruding head can seem like a periscope.

 

Northern Watersnake, Maryland
Creative Commons Attribution Share AlikeNorthern Watersnake, Maryland - Credit: Joby Joseph