This is what's known as Sou'wester
A practical hat used by seaman for protection against rough wet weather, The brim is wide and slanted basically designed to stop rain running off it down the collar of your coat.
A good chin strap also keeps it in place in high winds.
The name Sou'wester potentially comes from the South Westerly winds which brought rough seas, this may have some truth, but I can't find anything to back the claim up.
In some countries its just called a fisherman's cap, however they are not necessarily the same thing as as Sou'wester for example they are not needed to be fully waterproof.
Sou'wester are made of oilskin which can any be fabric made waterproof but is traditionally canvass covered in linseed oil or tar this one however is 100% PVC but works fine.
