There are few devices being used at once. It's an image that uses a simile, I.e. saying something is like something else. However, it's also a form of inverse anthropomorphism, in that it attributes a human quality to something non-human. "Hair like seaweed." The idea is to make a character (Unferth) seem connected somehow to the sea; bear in mind that Unferth at this point in the story is braggin... more