Question

The sea was like a fierce, black dragon. Is this an example of personification or simile?

Answer 1

Is it giving anything human characteristics?

Is it making a comparison using "like" or "as"?

The first is personification; the second is simile. What is your decision?

Answer 2

When it's using "like" or "as" it's a simile. When using no like or as it is a metaphor.
Example (simile): The grass was like a big jungle.
Example: (metaphor): The grass was a big jungle.
Personification is when something that is not human is given human-like qualities.
Example: Stars danced in the moonlit sky.
Danced is a human like quality, given to un-human things (stars).