This is a post for details/background information that couldn't make it into margaritas on the coastline (which will update with the final chapter some 30 seconds after I post this) so, spoilers beware! And a warning for canon-typical Anakin backstory, as per usual.
Anakin was also born in captivity in this AU. In a laboratory, to be exact. Most of his scars come from being experimented on, not from surviving in the wild.
Shmi did manage to escape with him when they broke out of the lab, but she died when he was about 14. Anakin prefers not to think about it. He'll never tell Obi-Wan willingly, but there's things in the sea that make Anakin look like a guppy, that humans have no idea about.
Anakin doesn't actually know how old he is, when his birthday is (Shmi didn't either), or how long it's been since any major event in his life. He just has a vague estimate/concept of time passing.
He speaks multiple languages, though fluency varies a lot. Largely correlated to how long he spent in the area, spying on humans. He's been in several different spots in the world. As long as there's an ocean, he can live there. Not usually as a "human"--that's kind of new.
Because he was lonely. And bored. He's never met any others of his own kind.
His type of merpeople need salt water to turn. He couldn't turn back during the night of the storm because there was so much saltwater flying through the air that he first had to find somewhere to dry off.
It's absolutely a painful transformation, but he's so used to pain between the torture and the normalcy of the change that he's just like, whatever.
However! He really doesn't like how sensitive his "new" skin is for a few hours after shifting. Obi-Wan learns quickly not to touch him anywhere but above his chest for a few hours after a shift.
His skin hardens and becomes pretty tough after just a few hours. You could still hurt him, but it's going to take effort, and the injuries will heal during the shift.
He does bleed red--but the consistency of the blood isn't quite... right.
The reason he was working was partly out of boredom and also out of half-concocted plan to buy himself forged papers. That's why he rarely spent any money on himself, he was saving it all up to get an identity.
He also just doesn't need a lot of stuff. He actually hates wearing shoes. But Obi-Wan gave him a gift, so he wears the stupid flip flops. Because the hot nice guy got them for him.
When he crosses the ocean (approximately 8000 km) he does so hauling a pretty heavy duty waterproof box where he kept cash he exchanged & a tablet with access to a crypto wallet for the rest. It took him a while to set up and make sure he could travel for months without losing it. He very much did not enjoy the experience, as normally when he moves around the world he's traveling with the warmth, not toward the cold. He is very grouchy about this and wants to be spoiled by Obi-Wan as a reward, thank you very much.
the stupid flip flops and the key are in the box. he knows it's stupid. he knows it's SO stupid. but obi-wan gave them to him. so he's keeping them.
Obi-Wan ends up moving to be with Anakin. Not exactly far, granted, but he still does end up moving to be with him.
Although Satine is not told The Secret, she's given enough details to grasp that Anakin technically doesn't exist. She pulls a few strings for them. (It's only fair; she kept the artwork and the flat. Also she's tired of Obi-Wan looking Sad And Worried whenever they meet to catch up.)