Coming of age
The Shinji that readers encounter at the beginning of the novel differs from the Shinji that is encountered at the end of the novel. Before Hatsue, Shinji lived a quiet, peaceful life characterized by certainty and naivete. He did not have ambitious life goals other than to own a fishing boat with his brother. He conceived of the sea around Uta-jima as a farmer conceives of his plot of land. He was rarely affected by any external forces, whether it was sickness or mental perturbation from some event or person. He felt uneasy when contemplating the unknown. However, this all changes with his love for Hatsue. He begins to have sleepless nights and experience new, disconcerting emotions. The drama of their love and courtship and its eventual frustration by Hatsue's father leads to his acceptance of a position upon the Utajima-maru; this is not done to impress Terukichi but to finally garner a taste of the unknown and of freedom. When Shinji returns from his voyage, he is more worldly and experienced. Even though he may not leave the island for good, he is content with the fact that he had actually been to the world beyond the horizon. This maturation allows him to also finally have the love of the woman he desires, completing the novel’s love story, for his strength and courage while facing the unknown are what win the approval of her father.
First love
The Sound of Waves is primarily known for being a love story. In his sparse, lucid prose, Mishima takes his readers through the courtship, estrangement, and eventual happy union of Shinji and Hatsue, two young denizens of the Japanese isle of Uta-jima in the post-WWII period. Mostly through Shinji's perspective, the reader comes face-to-face with the diverting, consuming, life-changing power of first love. Shinji himself is surprised at how he reacts to even the mere mention of Hatsue's name after he first sees her one day at the beach. He wonders if he is sick since he cannot sleep. He is confused and disconcerted at how he cannot seem to shake her from his mind. His once-simple plan of simply growing up and owning a fishing boat with his brother has not changed, but he can no longer conceive of this plan without Hatsue. Thus, his own coming-of-age is also inextricably linked with his taste of first love. The presence and thought of Hatsue excites and enervates his once-placid mind; he is more ambitious and desirous to leave the island and experience freedom due to this burgeoning relationship. Thus, first love is shown to be powerful as well as healthy and enlightening.
The power of nature
Nature's power is on full display in this novel. She is constantly revealing herself in the crashing of the waves, the pouring rain, the heaving seas, the tumultuous skies. The storms often take place at moments of heightened action or emotion in the novel, such as when Shinji must commit his act of valor on the freighter and when Shinji and Hatsue meet to express their love in the tower. Nature is supportive of the lovers' quest and makes no attempt to impede the fruition of their love; indeed, she can even be said to be guilty of subverting the forces of evil and immorality, as when a hornet stops Yasuo from raping Hatsue. Nature informs all aspects of life on Uta-jima and provides a daily living for nearly all of its inhabitants. As Chiyoko observes, the people of the island "enthusiastically entered into an alliance with nature and gave it their full support" (80). Nature is a mostly benevolent force that rewards hard work and morality. It provides, protects, and nurtures the island dwellers.
Knowledge vs. ignorance
There are salient differences between the characters that populate The Sound of Waves. While some of them are interested in expanding their ken, others are content to remain ignorant and live a small, simple life. In the former category exist Chiyoko and her mother. Chiyoko attends a Tokyo university and is interested in literature; she is frustrated with the dullness and backwardness of the island and desires to return to a place with nature is more subordinate and intellectual life more valued. Her mother, the lighthouse keeper's wife, has an insatiable desire to read and educate herself on a plethora of subjects. As for the latter category, characters like Shinji, Shinji's mother, Hiroshi, and even Hatsue do not seem concerned with the outside world or intellectual knowledge. Of course, Shinji changes his mind as the novel progresses, but he is still relatively limited in his encounters with the unknown—they are physical, rather than intellectual encounters. Hiroshi is absolutely uninterested in the wider world because it is uncomfortable and too thought-provoking. Overall, Mishima seems to pass little judgment on his characters, even the ones who are simple-minded and incurious. Knowledge and ignorance live side-by-side without any major problems, and ignorance of intellectual matters seems to leave more room for a close relationship with nature and the physical world.
Purity
The island is pure; it is not encumbered by modern technology or weighed down with human sin and depravity. There are no thieves on the island, and one might presume there are no other crimes either. It is beautiful and sustaining and timeless. Purity is also important as a personal characteristic that the two main characters, Shinji and Hatsue, possess. Indeed, the island itself seems to value the purity evinced by them and supports and rewards their courtship. Shinji is a paragon of purity; he is not cruel, selfish, prideful, idle, or gluttonous. Hatsue is also pure; she is polite, well-mannered, faithful, sincere, and perhaps most importantly, desirous of remaining chaste before she is married. The young couple do their best to live up to their strong moral standards and are rewarded at the end of the novel with the blessing of Terukichi and an engagement.
Mind vs. body
This tension is present within most of Mishima's novels, and The Sound of Waves is no exception; however, this tension is more or less resolved in the character of Shinji. He is a profoundly physical being whose greatest accomplishments are strength, endurance, and energy. He is described as being decidedly un-intellectual. When he ruminates on how to win Hatsue's favor, he realizes that he does not have much to offer other than these feats of strength. Indeed, it is his courage and his physical abilities that allow him to finally demonstrate his worth to Terukichi. However, as the novel progresses his mind does develop further; his love for Hatsue expands his ken and whets his appetite for experience. Shinji is becoming a fuller and more mature human being, capable of reconciling his corporeal and cerebral aspects. Neither his mind nor his body completes subsumes the other.
Social class
Social class is a significant aspect of the novel in that it keeps Shinji and Hatsue apart, thus forming the central conflict. Shinji is from a poor family and makes his living as a fisherman. Hatsue is from a wealthy family; her father, Terukichi Miyata, is a venerable and successful businessman who owns two coastal freighters. It is not altogether fitting that these two young people be allowed to marry, especially in Terukichi's opinion. This is especially important to Terukichi because he intends to adopt his daughter’s husband as his son, so that his name will still be continued even though his son has died.
Although the island is small and the living standards are fairly similar across class, the island dwellers live quite different existences depending upon their stature. Shinji's home is small and smelly, whereas Hatsue's home is larger and more comfortable. None of the rich families live in resplendent opulence (after all, they still have to draw their own water and none of their houses have private baths), but there are salient differences. It makes more sense that Yasuo should marry Hatsue, as he also comes from money. Nevertheless, social class does not prove to be insurmountable; Terukichi realizes that Shinji's exemplary character makes him a very suitable husband for his daughter, and that his lower social status is irrelevant.