The title of this excellent novel is of course rather curious, as no reef is actually refered to at any point in the text, and the action occurs well and truly on land and not anywhere near the sea. However, if we think of the reef as something that exists below the water, unseen and unnoticed, then we can relate the title of the novel to the way in which the truth about the relations between the three central characters is submerged for so much of the novel, only to come to light towards the end. Anna, towards the end of the novel, realises that "the truth had come to light by the force of its irresistible pressure." This also corresponds with her awareness of "hidden powers, of a chaos of attractions and repulsions far beneath the ordered surfaces of intercourse."
These quotes relate to both the truth of the relationships between the characters, but also the themes of constraints placed upon women, honesty and fidelity in personal relationships and the difference between verbal and non verbal communication. Anna, now that she knows the truth that she has been yearning for, finds it rather a unsatisfying dish, as she has to realise that there are parts of her new husband that will always be closed to her. The reef, representing the body of secrets and social realities that are always there, lying submerged, has been exposed, and Anna has to face the unsatisfying consequences of this.