Some specific fandom - dedicated forums might also be a good place to look for this type of fanfiction. These forums usually have sections for fan - created works, and if there are enough fans of Free! who like this concept, there could be some really interesting Rin x Haru merman stories posted there.
One common theme is the exploration of their relationship in a new, underwater world. Since they are mermen, it adds a different dynamic to their already complex bond.
One great Rin x Haru mermaid fanfiction is 'Under the Sea's Embrace'. It beautifully weaves the mermaid lore with their relationship. The author creates a vivid underwater world where Rin, as a mermaid, discovers a connection with Haru that goes beyond the ordinary. It has some really touching moments that show their bond in a unique way.
Friendship is a very common theme. Writers often explore how the deep bond between Rin, Haru, and Makoto affects their lives. For example, they might write about how they support each other through tough times in school or in their swimming careers.
I'm not sure specifically about lemon fanfiction involving them as it often goes against the general family - friendly nature of the original work. However, you can try searching on some fanfiction platforms and look for well - written non - inappropriate stories about their relationships.
Swimming is also a big theme. Given that they are all swimmers, many stories will involve them training, competing, or having some kind of revelation related to their swimming careers. For instance, a story could be about how Rin's new training technique inspires Haru and Makoto to improve their own skills.
One common theme is the exploration of their deep emotional bond. Since they have a complex relationship in the original story, in the Yaoi fanfictions, this bond is often intensified and turned into a romantic or sexual relationship. For example, they might struggle with their feelings for each other while also dealing with external factors like their swimming careers.
One common theme could be care and concern. For example, if Rin is sick, Haru might show his worry in a very characteristic way, like being more protective. Another theme could be vulnerability. When sick, the usually strong - willed characters become more fragile, which can add an interesting dynamic to their relationship.
One common theme is the exploration of their deep emotional connection. Since they are often portrayed as having a strong bond in the original work, fanfiction may delve deeper into how that bond could develop into a romantic or more intimate relationship in a Yaoi context.
You can start by looking on popular fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fan - created stories, including those about these characters. Just make sure to filter the content according to your preferences.