His water - based abilities would likely be more refined. Amphitrite could teach him advanced techniques for controlling water, like creating complex water formations or using water for long - distance communication. He might also have a better understanding of the magic within the sea, being able to tap into hidden powers that normal demigods wouldn't know about.
His combat skills could be different too. Instead of relying mainly on his sword fighting, he might be more proficient in using water as a weapon in a more creative way. He could form water blades that are more powerful and durable than the ones he normally creates. Also, his ability to heal using water could be enhanced, as Amphitrite might teach him ancient sea - based healing methods that are not known to most.
Percy might develop abilities related to sea life in a more profound way. For example, he could have the power to communicate with all types of sea creatures, not just the ones he could in the original story. He could also have the ability to manipulate the environment of the sea floor, such as creating or destroying underwater structures with ease. This would be because Amphitrite would expose him to more of the secrets of the ocean's power.
His chakra control would probably be better. In a temple, he might learn meditation techniques that help him master his chakra more effectively. Also, his physical abilities could be more refined in terms of balance and grace, like a monk's movements.
Naruto's combat style would be more refined. Sakumo was a highly skilled ninja, and he would teach Naruto better swordsmanship skills perhaps. Naruto's taijutsu could also be more precise and calculated. Also, Naruto might develop better chakra control earlier, as Sakumo could instill in him the importance of efficient chakra usage from a young age.
His physical abilities would be enhanced. Constantly running, climbing, and fighting in the forest would make him stronger and more agile. His stealth skills would be top - notch as he'd have to hide from the forest's predators.
Her Semblance could be more destructive. Instead of using it mainly for mobility and quick attacks, it might be used to cause widespread chaos. For example, instead of just dashing in and out of battles, she could use her speed to create tornado - like effects that cause more damage to her enemies.
Mentally, Harry would be more in tune with the Force. This would give him an edge in sensing danger and the intentions of others. His ability to resist the Imperius Curse might be stronger due to Jedi mental training. Instead of using spells for divination, he could use the Force to foresee events. And his telepathy - like abilities from the Force could be used in place of some magical communication spells.
His abilities in alchemy would be far more advanced. He'd likely be able to create potions and elixirs with ease, having learned from Flamel. Also, his understanding of magical objects, especially those related to the Philosopher's Stone, would be greater.
Naruto's ability to control the Nine - Tails would be enhanced. Since the Sage has a deep understanding of the tailed beasts, he could teach Naruto how to fully harness the power of the Nine - Tails without the usual struggle. This would make Naruto not only stronger but also more in tune with the beast's power, allowing him to use it more effectively in combat. He could also potentially communicate with the Nine - Tails on a much deeper level, leading to new forms of cooperation between them.
Percy's personality would probably be more solitary at first. Growing up with wolves, he's used to relying on himself and his pack. So when he enters the world of demigods, he might have trouble trusting others immediately. He'd be more in tune with his instincts, which could make him seem unpredictable to those who don't understand his background. For example, he might follow his nose or gut feeling more often than following the advice of others.
Naruto could have a greater control over his chakra. Since Kushina would be there to guide him, he would likely avoid some of the pitfalls he faced in the original story. His physical strength might also be enhanced as Kushina could train him in ways that utilize his Uzumaki bloodline more effectively.
If Naruto was raised by Slenderman in fanfiction, his abilities would change drastically. His chakra control might be influenced by Slenderman's otherworldly energy. He could gain the ability to manipulate the environment in a more spooky way, like making the air feel cold and heavy around his opponents. Also, his Rasengan might take on a different form, perhaps with a dark, swirling energy around it.