While you didn't provide a bolded line, I can infer which sentence was originally bolded.
The figurative language device used in the bolded line is "simile." A simile is a figure of speech that compares two unlike things using "like" or "as." In this case, the author compares the flatness of the sea to a "plate-glass window" to convey the smooth and motionless nature of the water.
I hope this helped!
~~~Harsha~~~