In the new Aquaman movie there are a number of scenes where Atlantians are seen on land wearing suits filled with water so they can breath. In some of the fight sequences we even seen their suits breaking and the person inside "drowns" like a fish out of water. Yet Aquaman, his mother, and numerous other Atlantians have no trouble breathing both air and water. I suppose in Aquaman's case he was half Atlantian and half human, but that doesn't explain the others who had this ability. I felt like this was either not explained at all or else I missed it. Why do only some Atlantians need water breathing suits while others don't?
It's class/genetically based.
Here's an excerpt from an article i found.
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