In heraldry, a line which is drawn wavy is made up of a series of waves. Not surprisingly it is often used to represent water.
Certain charges using the wavy/water motif have their own names. A roundel barry wavy azure and argent is called a fountain, and a base barry wavy azure and argent is called a ford.
Wavy may also be indicated by the term Undy (also spelled Ondé, Oundy, Undé).