Fale morskie and other 86,400 icons from Icons8 icon pack follow the visual guidelines of the operating systems: Windows, iOS, Android KitKat, and Material. PNG format is free up to 100x100 px. Icons are availble for download in PNG and vector (SVG, PDF, EPS, and more).
The icon for sea waves is three lines that are drawn to represent the up and down motion that waves make when the ocean water is moving. The three likes are placed one under the other, because that way it looks like a body of water.