Symbolism, Imagery, Allegory
The sea is a natural vehicle for force, change, and motion. We think the novel wants you to see that even in the natural world, things change. The sea moves, it upsets things, and it's both dangerous and serene. We'll let Charlotte do the talking:
As for the sea, it was almost the same color as the sky, a menacing claylike gray. And yet, it was in constant motion, its surface heaving rhythmically like the chest of some vast, discomforted sleeper. (4.3)