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.
I would concur in that the statement is an example of personification. The idea behind personification is being able to give human characteristics to objects or things. The idea of "cradling" is something that a human does to an infant, and the wave, being an object, is being personified here by cradling the swimmer. Personifications works and is successful on many levels. It helps to bring out a sense of imagery by allowing the reader's imagination to better envision the conception of a particular entity in giving it human characteristics. Another way in which personification works is in its empathetic manner, an attempt to allow sensitivity to extend to objects. The reader sees them as more than as such when they are given human characteristics.
This is an example of personification. Waves do not possess arms. Personification is giving human characteristics to something nonhuman, and in this case the writer gives arms to waves.
Metaphor can be viewed as an umbrella term that really encompasses all figurative language, since all figurative language involves comparison. Personification does, of course, involve an implied comparison, so it, too, falls into the category of metaphor if metaphor is used in the broad sense.
But, strictly speaking, the quote you give is not metaphor. Of course, it is not simile or alliteration, either.
This is an example of personification. Personification is when you say that something that is not alive (or not human) has characteristics of something that is alive (or is human). So, in this case, you are saying that the wave has arms and can cradle someone. This is not true of waves, but of course humans have arms and can cradle other people in those arms.
I suppose you could argue that this is a metaphor because the wave is being compared to a person, but I don't think it is. I think a metaphor would actually say what it is comparing the wave to, like in "the wave was a mother and cradled..."