One possible fanfiction could be about Peter Parker being bitten by the radioactive spider while on a school trip to the beach where he encounters a mermaid. The mermaid could be a witness to his transformation and help him understand his new powers in a unique way. Maybe she has some special knowledge about the spider's origin that ties to her underwater world.
In a mermaid Peter Parker fanfiction, the mermaid could become Peter's love interest. Peter, being the kind - hearted hero, might save the mermaid from some sort of danger. Their relationship could be a secret, and he would have to balance his life as Spider - Man and his new relationship with the mermaid. This could add a whole new layer of complexity to his character and the story.
A mermaid Peter Parker fanfiction could also explore the idea of Peter using his web - slinging abilities underwater. The mermaid could teach him how to move more efficiently in the water, and they could team up to fight underwater villains. This would be a really cool take on the traditional Spider - Man story, taking him out of his usual urban environment and into the underwater realm.