When searching for the themes of a novel, it may be helpful to identify topics that are addressed, and then ask yourself, what kind of message is conveyed about each of these topics in the novel? For example, in Whale Talk, one topic that may be identified is identity. Many of the characters in the novel struggle with forming identities with which they feel comfortable. T.J. finds it difficult to truly identify with any of the ethnicities which make up his mixed background. He also finds it difficult to be accepted in society because of the strong presence of racism not just in his hometown, but in the entire country. Chris Coughlin is another character who struggles with identity. He is mentally handicapped and he clings to the identity of his older brother who has passed away. He does so through his letter jacket. However, the jocks of the school criticize Chris for wearing a jacket he has not "earned". This injustice is what leads T.J. to form a swimteam in which the outsiders of the school find a place to feel accepted. So, what is the statement about identity? You might say that one must take a journey to find identity and that journey needs others to guide the way. People find out who they are through the help of the perspectives of others. Everyone needs some kind of positive encouragement to be confident in his or her identity.
The theme of identity also connects to the theme of the universality of human existence. The title refers to a video watched by T.J.'s father of whales communicating. T.J.'s father compares this to human communication. Though all humans may need to hear the same messages, he feels as though some never do.
I hope this helps. To find other themes, try to identify topics or issues addressed throughout the entire novel, and evaluate the overarching statement about each one.