At the end of the novel, Joy is at home again. She imagines what it would be like to learn to swim. She has read somewhere that
the trick is to keep breathing and
it comes with practice. This is central to the whole novel.
trick suggests that there is a knack or technique Joy can learn to see her through her mourning, depression and life in general. It could be read as a positive, playful statement that is symbolic of her will to live and to survive.
The image of swimming is also important because it contrasts with Michael's drowning, and Joy's mother's death. It is a statement of affirmation: Joy will not similarly drown in her depression.
Joy learns to accept herself. She no longer takes on the blame for everything and manages to say
I forgive you to herself. This suggests that she is on the road to recovery.