Love across species is a common theme. Sea dragons are often depicted as different from humans or other sea creatures, but love still blossoms between them. Another theme is adventure. Since sea dragons usually live in a mysterious underwater world, there are often adventures involved in the stories.
One of the main themes is the power of love to bridge differences. Sea dragons can be seen as very different in nature, appearance, and abilities from the other characters in the story. However, love has the power to bring them together and make them understand each other. There's also the theme of environmental connection. Since they live in the sea, these novels often touch on how the sea dragon's world is affected by environmental changes and how the characters fight to protect it while also nurturing their love.