Final answer:
The simile means that someone's emotions are as unpredictable and mysterious as the ocean; they can be occasionally understood but are often difficult to read.
Step-by-step explanation:
The underlined simile in the quote from "The Lightning Thief" compares the difficulty in reading someone's emotions to the unpredictability and mysteriousness of the ocean's moods. It suggests that just as the ocean can sometimes be perceived as calm or stormy, a person's feelings can occasionally be understood. However, much like the deep and often-obscure ocean, a person's deepest emotions can be difficult to decipher, remaining mysterious and unreadable.