In sharkboy and lavagirl love fanfiction, the theme of self - discovery through love is often seen. As they interact and fall in love, they also learn more about themselves. Also, the theme of unity is popular. Their combined powers along with their love can be a symbol of unity, and fanfictions might explore how they use this unity to make their world a better place or to defeat enemies. There could also be themes related to family acceptance, as their relationship might face challenges from their families or the people around them in some stories.
A frequent theme might be the discovery of their feelings for each other. In many fanfictions, they start as friends or comrades in their adventures, and then gradually realize their romantic feelings. Another theme could be sacrifice. They might be in situations where one has to sacrifice something for the other, and their love is what makes that sacrifice meaningful.
One common theme could be the power of their love overcoming challenges. Since they both have unique powers, there might be stories where their love helps them face external threats.