Here's why:
* Simile: A simile uses the words "like" or "as" to compare two things.
* Metaphor: A metaphor directly states that one thing is another, without using "like" or "as".
Since the sentence doesn't use "like" or "as," it's not a metaphor. It's comparing the sound of the sea to the sound of a roar, using the word "like" to make the connection explicit.