É de classe / baseada em genética.
Aqui está um trecho de um artigo que encontrei.
While it involves genetics, it also seems to be a matter of class standing. Through the film's dialogue, it's explained only highborn Atlanteans are able to breathe both air and water. That means royalty and anyone who associates with them on a regular basis.
- Aquaman Movie Explains How Atlanteans Survive on Land