One possibility is that he discovers an ancient, hidden grove that has a powerful magical spring. This spring could have the ability to heal any wound or grant temporary superhuman strength. Percy would then have to protect this grove from other forces that might want to use it for evil purposes.
He could encounter rare mythical creatures that are only found in the deepest parts of the wild. For example, he might come across a group of centaurs who have a different culture compared to those he met before. These centaurs could teach him new survival skills and ancient lore.
Percy could discover ancient and forgotten temples or shrines in the wild. These could hold powerful artifacts or be the lairs of minor gods or spirits. He might have to face various traps and puzzles to explore them fully. In the process, he would rely on his skills as a demigod, like his ability to control water, to overcome obstacles and uncover secrets. This would be an exciting addition to the fanfiction as it would add new elements to the Percy Jackson universe.
He would become more self - reliant. In the wild, there's no one to rely on but himself most of the time. So he'd learn to trust his instincts more. For instance, when it comes to finding food or water, he'd have to quickly figure out what's safe to eat or drink.
One possibility is that Naruto stumbles upon a hidden village in the wild that has been cut off from the rest of the ninja world for centuries. He has to learn their unique culture and way of life. This experience could shape his view of the ninja world in a new way. He might also find a powerful and mysterious mentor in the wild who teaches him skills that are different from what he would learn in Konoha, such as how to use natural elements in a more primal and instinctive way.
He could have adventures involving other sea - related deities. Since Thetis is a sea - goddess, he might interact more closely with her kin. For example, he could be sent on a quest by Thetis to retrieve a powerful artifact from the deep sea that is guarded by a fearsome sea - monster.
He might encounter magical creatures that are not known to most wizards. For example, he could meet a rare breed of unicorn or a group of centaurs who take him under their wing and teach him their ways of magic and life in the forest.
In the fanfiction, living in the sea could give Percy a chance to explore the secrets of his water - related powers more deeply. He could learn to control the tides in a more precise way or communicate with the very essence of the ocean. He might also face challenges such as dealing with underwater magic that is different from what he's used to on land. There could be ancient sea spells that he has to decipher in order to save the sea from a dark force.
They could have an adventure where they have to retrieve a lost artifact that is important to both their parents. Percy could use his ability to control water to navigate through underwater caves or obstacles, while Annabeth's knowledge of architecture and strategy would help them figure out the puzzles and traps along the way.
Well, he might discover hidden caves filled with ancient treasures. Maybe he'll meet some unique dwarven craftsmen who teach him about their art.
Perhaps he has been exposed to some sort of deadly and unknown substance in the mythical world. This could be a new type of monster venom or a strange plant that he accidentally ingested. The substance could be attacking his body from the inside, causing his internal organs to bleed, and thus he throws up blood.