"The wave cradled the swimmer in her arms" is an example of which literary device?

I agree that it's personification, but you could also call it an example of "pathetic fallacy."  Here, 'pathetic' refers to empathy, so it doesn't really mean pathetic. Pathetic fallacy is a broad type of personification. 

Pathetic fallacy is also called the anthropomorphic fallacy. It's when writers give inanimate objects (often nature) human qualities, emotions, feelings, thoughts and so on.  So the wave is given qualities like a human. Most obviously, giving the wave 'arms' is an example of personification and the pathetic fallacy because the wave cradles the swimmer as a lover would or as a mother would cradle a child.

Just going back to the more broad concept of pathetic fallacy (and this might make it seem pathetic), is also the idea that inanimate objects, particularly nature, have human empathetic qualities. Think of the typical hollywood movie when a couple breaks up. It's usually raining. This has become such a cliche that it borders on pathetic - as if nature is grieving with the character.