They would probably start off being a bit cautious of each other. Harry would be intrigued by Arthur's unique powers and his connection to the ocean. Arthur, on the other hand, would be curious about Harry's magic. They might exchange stories about their respective worlds at first, like Arthur telling Harry about Atlantis and Harry sharing tales of Hogwarts.
Maybe Arthur Curry is sent to Hogwarts on a mission. He has to learn magic quickly to blend in and help solve a mystery that involves dark wizards trying to steal Atlantean magic. Along the way, he befriends Harry Potter and they exchange knowledge - Arthur teaches Harry about the ocean and its secrets, while Harry shows Arthur the ropes of wizardry.
Love and attraction are also quite common themes. How the reader and King ORM fall for each other despite their differences in origin and status. There's also the theme of adventure. The reader is usually thrust into a world full of magic or power struggles, and they embark on an adventure with King ORM by their side. For example, in many stories, they go on quests together to save the realm or solve some mystery. It adds an element of excitement and unpredictability to the story.
If Orm had succeeded in such a fanfic, it would likely trigger a chain of events that would test the limits of the remaining heroes and their resolve. It might also bring out unexpected alliances and power plays within the superhero community.
In the main comic storylines, Aquaman is not gay. However, there might be alternate universes or special editions where his character could be portrayed differently, but that's not the norm.