What are the motifs in The Sea, the Sea by Iris Murdoch?

Marriage is a recurring idea in the story. The nature of marriage is referred to several times throughout the book. It's spoken of with general contempt by Charles, but with general respect by several of the other characters, particularly Titus, Peregrine and Hartley. In their various ways, they repeatedly tell Charles that he doesn't understand what marriage means, that even if a marriage seems difficult to him it may function perfectly well from within, and that he must respect the will and desires of those within the marriage.