Here's how I see him...
Atlan was a wily old wizard-- like Flagg in Eyes of the Dragon, only not so malevolent. In the DC Order/Chaos scheme of things, call him an ambassador from the realm of Chaos. He doesn't much care whether society crumbles or if good triumphs or a revolution succeeds-- but, when you get down to it, he's a smart fucker, and he likes other smart fuckers, and tends to root for them. He's thousands of years old-- at least-- and he wanders all over Earth and the mortal planes in general, but something keeps pulling him back to the oceans...
Thetis is basically Ariel with legs. (In my DC Atlantis, fuck mermaids/men and Lagoon Boy and all those Atlanteans who look like Triton from the Inhumans.)
She longs to know what the surface world is like. Her husband, King Nereus, is a generation older than her, and only interested in her for one thing-- and even then it's only for about an hour a week. She dreams of going to the surface and being treated like a real queen, not the whore with a crown that she feels like. She's the youngest of her family, one of the few noble families left in Atlantis. She had the choice of marrying the king or seeing her family sell off the last of their estates to avoid slavery for one more generation. Lord knows what they find to keep slaves busy down there.
Atlantis itself is the largest of a group of warring city-states, sharing a similar background but devoted to different ideals (think the pre-Alexander Mediterranean). They inhabit the Northern Atlantic, though they know there are other cities in other seas. They've been fighting the same stupid battles and using the same ancient magic for three thousand years or more, since they were, for lack of a better explanation, kicked out of the Mediterranean for fucking everything up (I'll figure out a better why later). Every time Atlan swings by he stirs up trouble, and every time they just rebuild everything just as it was and go back to the same genealogy-worship and Hatfield-McCoying and slavery and sieges. Think of the Inhumans and their lost city of Attilan. It's kind of like that down there.
Nereus is just another king. He's kind of an asshole, but he'll repent later, after Thetis is dead. He fucks around a lot, both with his literal penis and the figurative penis of his royal authority. That'll come back to bite him.
Atlan takes a liking to Thetis. He promises to take her to the surface world. He pretends that she needs his magic to breathe air, and lets her run around on land for a night and a day. Perhaps she meets Tom Curry, a lonely lighthouse-keeper? Perhaps.
It comes time for Atlan and Thetis to return to the palace. Thetis is giddy from her adventure, Atlan thrilled to have a new pawn to wreck things for Atlantis... the guards catch them. Nereus is an asshole, but he's not an idiot. Thetis is put under house arrest. The king's guards try to capture Atlan... in his own words, "HA!" In a flash of light and a puff of sulfur, he's nowhere to be found (not that that means he's left the premises, necessarily).
Nine months later... screaming in the Queen's quarters. Sounds of violence. The king demands to know-- is she alive? Does he have an heir?
Yes and no. The child-- the boy-- is alive. The queen went into convulsions and her hair turned white as she pushed him from her body. Now she lays still, catatonic. The boy has blond hair and green eyes. Like Atlan, the wife-stealing sorceror. Like Gamemnae, the witch who caused Atlantis to sink. Tainted by the outside. By the surface. Better if he were stillborn. So they take him to Mercy Reef and leave him to die. Herein they make the classic villain's mistake of "I'll just leave you here to suffer and die. I don't need to actually witness your death myself. I trust you." Trust not the dolphin, ye below the waves.
A pod of dolphins rescues the baby, who soon learns to swim and communicate with his new family. For years, young Orin is happy to be the Mowgli of the Atlantic... although how a human child manages to grow up without serious problems after spending most of its formative years eating nothing but dolphin milk and raw fish is beyond me. We'll file that one under "magic."
Orin is part of the tribe, so to speak, until sometime around the age of seven. Then, while he and a "brother" are hunting a particularly speedy school of fish, a terrible hurricane separates them from the rest of the pod. They're dragged to shore. His brother beaches and dies. Orin is found by, you guessed it, Tom Curry.
I see Tom as a loner type with a good heart. Maybe for awhile there are rumors around his beach of a "wild boy" causing trouble. Maybe he finds Orin and thinks he's a runaway and just tries to be a mentor to him at first. Soon, though, I see Tom taking Orin in, teaching him English, and this is the important part, putting him in public school. Soon he's calling Tom Dad, doing homework and making friends. By giving Arthur the trappings of a normal (ish) American upbringing, we make it easier for readers to relate to him. Tom puts in a call to an old friend from the Navy (of COURSE he was in the Navy) and sets up a Social Security card and Birth Certificate for his new son. Who is Tom Curry? Besides the former Navy conscription and current lighthouse-keeper job, it's pretty wide open. Maybe he's a curator of the lighthouse, a historian type. Or maybe he, well, saw some shit when he was overseas and is all bitter and grim now and he wants to live in the lighthouse, away from people. Could be we take the Golden Age route and he's a crazy marine biologist with dreams of playing God (or at least Neptune) with genetics. Lots of options. But back to Arthur.
So Arthur gets to go through a semi-typical adolescent period, first worshiping the ground Tom walks on and then being a rebellious teen. He wants to know where he came from... too bad Tom honestly has no idea. Unless it's that strange woman he met all those years ago? Tom does the best he can, but whatever his past was, he's not great with people, let alone kids. Arthur gets into a lot of trouble in high school, he fights with Tom a lot, and spends a lot of time swimming/surfing/stealing boats/whatever. But he still doesn't know who he is. At this point, his memories of life before land/Tom are blurry at best. His inherent telepathy/affinity for magic/regal charisma make for some interesting times at school, though, where a gang of followers gradually accrues around him and a lot of the girls (and some boys of course) see him as the town's hottest bad boy.
I could go on forever about the Adventures of Young Arthur in America, but let's cut to the chase. Tom Curry isn't a young man... well, I started thinking about it and I'm wondering how much tragedy needs to be injected into an origin story. He was abandoned by his birth parents. Then he was separated from his adopted dolphin family. Tom dying is a foregone conclusion... do I need to Geoff-Johns it up and see Tom murdered or is a heart attack or something enough?
Either way, Tom dies and Arthur leaves... for where? Another place where I get a little fuzzy. Does he head straight for the ocean? That certainly works... but I also really like the idea of him wandering the Earth for awhile, having adventures. This would also be where he meets Kako the Inupiat girl and fathers Koryak, his "evil" son, not to mention running afoul of other young soul-searching world travelers like, say, Bruce Wayne, Diana Prince, or Clark Kent. Just an idea. But sooner or later he's going to run afoul of Atlantis... and slavery, and wisdom, and redemption.
2 months ago