It's possible that his powers could even be enhanced in some ways. Losing an eye could make him more in tune with his other senses, and this could translate to his powers. For instance, his ability to sense danger through water vibrations could become sharper as he has to rely more on it. His powers might also develop in new directions. Maybe he could start to use water to create illusions to compensate for his lost vision in battle, like using water to project false images of himself to confuse his enemies.
One possible consequence could be a change in his fighting style. Without his eye, he might have to rely more on his other senses like hearing and smell, similar to how blind ninja in the Naruto universe adapt. For example, he could focus more on using his chakra to sense the movements of his opponents.
One consequence could be that he might isolate himself from his friends. In fanfiction, Percy is often depicted as a social and heroic figure. Losing hope could make him feel like he's a burden to others and cause him to withdraw. For example, he might stop showing up at Camp Half - Blood activities.
He would become more cautious. Losing an eye would be a harsh reminder of his mortality. He'd likely train harder to make up for the loss of vision in one eye. For example, he might practice more with his sword in the dark to get used to fighting with limited sight.
One consequence could be that the fanfiction community might feel less connected to the character. If Percy doesn't seem to interact with or acknowledge the fan - created stories, fans might feel like their creativity isn't being recognized. For example, they might be less motivated to write more stories about him.
It could lead to a breakdown in the demigod community. Percy is kind of a hero figure, and if he doesn't care, it might demoralize other demigods. They might start to question their own purpose and the importance of their roles. Also, it could make Camp Half - Blood more vulnerable to attacks as Percy is a strong fighter.
One possible plot could be Percy going on a solo quest specifically designed to help him master his powers. He might face challenges that force him to use his water - based powers in new and extreme ways, like having to freeze an entire lake to stop a monster from escaping. Along the way, he could meet a new mentor, perhaps an ancient water spirit, who teaches him secrets about his abilities that he never knew before.
One possible consequence could be a significant change in his fighting style. He might have to rely more on his left side and develop new jutsu techniques that can be executed with one arm. For example, he could focus on enhancing his Rasengan further to compensate for the loss of his right arm's abilities.
One possible consequence could be that he becomes even more powerful in a dark way. Tartarus is full of dangerous and ancient forces. Staying there might expose him to unique and powerful magics that change him. For example, he could gain new combat skills or the ability to control certain Tartarus - born entities.
Well, one possible consequence could be that Percy gains an immense amount of power. Kronos was a very powerful Titan, so Percy might become nearly invincible. He could also face some internal struggles as Kronos' essence might try to influence his mind and actions.
The entire world of the story could be altered. Monsters might be stronger or weaker depending on what he changes in the past. If he stops some key event that was related to a monster's origin, it could lead to a different power balance in the world of demigods.