Given that the novel is set on the coast of an island in a town whose primary industry is fishing, it makes sense that the sea figures as a major character. Proulx and her characters often personify the sea, sometimes in positive ways but often in negative or even villainous ones. Despite its beauty, the sea is responsible for many of the story's darkest events, including the death of Jesson, the eldest Buggit child, and the near deaths of Jack Buggit and Quoyle.
In Chapter 13, Bayonet Melville, the white-haired owner of Tough Baby, describes how "the sea absolutely went mad," resulting in Tough Baby's destruction of 17 boats. Like people, the sea exhibits a range of moods. Sometimes it gets angry or frustrated, whereas other times it is calm. Its presence, though, is consistent, like in this example from Chapter 33:
The house was heavy around him, the pressure of the past filling the rooms like odorless gas. The sea breathed in the distance.
With all the change coming to Killick-Claw, the water is a constant, even if its behavior is unpredictable. The sea is not the only natural phenomenon Proulx personifies: she often mentions the shrieking winds and sagging sky as well.