As the previous poster noted, one of the main themes in the story is that of the sea. Because her sons are always leaving her to be killed by the sea along with her husband, Maurya has become terrified of its power and wants desperately to find a way to save her two remaining sons from its power. In the end, however, she cannot do so and the final two end up drowning and the incredible power of the sea both as an attraction and a destroyer is made evident.
Another theme is that of loss and the inevitability of it. Of course her sons and her father did not have to go to sea and not everyone who goes to sea drowns, but Maurya finds a sort of fate in the power of the sea over her family and her dreams and her waking life are haunted with the power of the sea to reach out and take away her most precious possessions.
The main theme concerns the power of the sea. As an island, Ireland has a very powerful connection to the sea. In many ways, the sea is both provided and destroyer. It can be as much of a boon as a hindrance. The only connection that the inhabitants have to the mainland is through the sea, and in that way it provides life, but the sea also has the power to take life as well. Although set in contemporary Ireland, the novel gives a glimpse into the connection that generations of Irish have had to sea and the power it had over their daily lives.