Sophie, and her Dad, both describe her as being like the ocean. Her father calls her "three-sided Sophie", and then Sophie states that she has "sides like the ocean".
These 'sides' are the different personalities of the ocean. One day it can be calm, serene and beautiful, as far as the eye can see; the next it will be turbulent, all-powerful and almost flippant with life; finally, the ocean can be breezy and playful, but not altogether dangerous.
Sophie says she is not "always such a dreamy girl, listening to the sea calling me". The 'face' Sophie puts on in different situations is only one 'side' to her character. Many people have this (or similar) personality characteristic - for example, being boisterous around friends, but shy around strangers.
This underlying turmoil, and even untrustworthiness, of the ocean (and Sophie) is reinforced by swapping the narration between Cody and Sophie. Can we always trust her version of the story? Can we always trust the ocean to keep us safe?