Well, he might start by being really confused. The Devil Fruits would be so strange to him compared to chakra. He could probably make friends with Luffy easily though, since they both have that adventurous spirit. And his ninja skills could come in handy when fighting pirates. Maybe he'd teach some of the Straw Hats some cool ninja moves.
In a fanfiction where Naruto is a fishman in One Piece, Naruto would face new challenges. For example, dealing with the discrimination that some fishmen face in the One Piece world. But he'd also have new opportunities. He could use his fishman physiology to swim faster and deeper, discover hidden treasures, and maybe even find a way to connect the ninja world with the pirate world. His Nine - Tailed Fox chakra might also have some interesting interactions with the strange powers in One Piece.
If Naruto were in the One Piece world, he could form interesting alliances. He might team up with Luffy and the Straw Hats. Naruto's ability to make friends easily would fit right in with the Straw Hat crew's spirit. His Rasengan could be a very powerful attack against some of the strong foes in the One Piece world. Also, his experience with dealing with different types of enemies in his own world would give him an edge in strategizing in the new and strange world of One Piece.
Naruto's ninjutsu skills could be very powerful in the One Piece world. His Rasengan, for instance, is a concentrated ball of chakra. In the One Piece world, it could be as strong as some of the more powerful Devil Fruit attacks. His ability to run up walls and trees using chakra control could also be useful in the more diverse terrains of the One Piece world, like running up the side of a ship or a rocky island.
Naruto could fit in by using his physical abilities. His superhuman strength, speed, and endurance, which he developed through his ninja training, would make him a formidable opponent in the One Piece world. He could easily take on weaker pirates or even some of the lower - level Marines.
Kankuro could use his puppet - making skills to his advantage. He could create puppets that are resistant to the elements in the One Piece world, like water - proof puppets since there are a lot of sea - related challenges there. Also, he could make puppets that can fly to deal with the various islands and different terrains.
Another concept could be that the Nine - Tailed Fox from Naruto somehow gets loose in the One Piece world and its chakra affects Devil Fruits, causing strange mutations. Luffy and his crew then have to team up with Naruto and his friends to find a way to stop the chaos.
Well, she might be really confused at first. The magic in the Harry Potter world is quite different from the powers in One Piece. She could try to use her spells, but they might not work as expected. Maybe she'd meet Luffy and his crew, and they'd be really curious about her strange clothes and the wand she carries.
Naruto's ability to create shadow clones would be extremely useful in the One Piece world. He could send clones to gather information across different islands while he and the Straw Hats are on their main journey. His Rasengan, being a powerful concentrated attack, could be used against tough enemies like the Yonko's commanders. And his immortality means he can keep using these abilities without the fear of death, making him a very formidable ally in the One Piece universe.
Naruto's Rasengan could be seen as a super - powerful attack similar to some of the advanced Haki techniques in One Piece. His ability to create shadow clones would be a huge advantage in battles and also for things like scouting or distractions. His chakra control could potentially be related to Haki control as well. And his immortality would make him a very unique and feared presence in the One Piece world, as most characters there are not immortal.