Naruto would be very curious. He'd want to understand what's going on. Once he figures out that there's a big battle between different factions, he'd try to find a peaceful solution. He might start by talking to the leaders, using his charm and persuasion skills. If that doesn't work, he'd use his ninja powers to protect the innocent. He's not one to just stand by when there's injustice.
If Naruto arrives at Marineford, it would be a chaotic and interesting scenario. Naruto's powerful jutsu like Rasengan and his ability to summon toads could change the balance of power. He might try to stop the fighting between the pirates and the marines, seeing the senseless destruction. His sense of justice and never - give - up attitude could inspire some of the characters there.
He might be surprised. He's used to his own life and adventures, and seeing it twisted and turned in fanfiction could be unexpected.
Naruto might have a mixed reaction. He could laugh at the funny parts of the fanfiction. However, if the fanfiction misrepresents his friends or his village in a negative way, he might get angry. He's very protective of those he cares about. He may also be intrigued by the different versions of himself that are presented in the fanfiction, and might even start to think about how he can grow in those new directions.
One consequence would be a major shift in the power balance in the One Piece world. The Marines would be severely weakened, and pirates might become more bold. Another consequence could be that Naruto would become a huge target for both the remaining Marines and other powerful forces in the One Piece world.
He would probably be very confused at first. Naruto has always trusted Tsunade, so the betrayal would come as a huge shock.
Naruto would probably be very surprised. He has heard about Rin from Obito and Kakashi. So when he actually meets her, he might be at a loss for words at first.
He would be heartbroken. Naruto values friendship above almost everything. At first, he might not believe it and try to reason with his friends.
At first, Naruto's eyes would widen in surprise. He'd pick up the fanfiction, not really knowing what he's getting into. As he starts to read, he'd be constantly muttering things like 'What the heck?' and 'This is so strange!'. He might try to find someone to explain it to him, but if no one could, he'd likely just shrug it off as something he doesn't need to understand.
He could initially go into denial. Naruto has always been an optimist. He might not be able to believe that his wife, the person he loves, could betray him. But once the truth sinks in, he would start to analyze the situation carefully. He might try to find a way to redeem her, depending on the nature of the betrayal, as he always believes in the power of forgiveness and second chances.
Sakura would probably be very angry at Kurenai. She's protective of Naruto and would demand an explanation. She might even try to heal Naruto right away if it was a physical injury.