Most Atlanteans in the DC Comics universe may look just like humans, but their anatomy is obviously quite different, as evidenced by their ability to breathe underwater. Atlanteans are also capable of breathing air, and in the comics, the transition from underwater to surface isn't anything major for them. However, the Aquaman movie decided to take a different approach with this by having the Atlanteans needing to vomit whenever they start breathing air. As director James Wan explained, this is meant to get the leftover water out of their lungs so they can take in air without any issues, stating:
Just from a practical standpoint, people ask me, when they talk, 'Is it bubbles that come out of their mouth?' No, 'cause there's no air in your lungs, so there wouldn't be any bubbles, right? Because you don't do that, right? And, so, then, in that sequence, where you saw Mera open up this air pocket to suck Orm away from Arthur, help him out, now, when he's breathing, the first thing he does is he using the air pockets, where he could get out all of the water that's in his lungs.
To provide some context, James Wan gave this explanation to Comicbook.com and other outlets during their visit to the Aquaman editing bay after showing a sequence where Amber Heard's Mera rescues Jason Momoa's Arthur Curry from his battle with Patrick Wilson's Orm, a.k.a. Ocean Master. She does so by opening up a patch of air underwater, and when Orm lands inside of it, he's briefly incapacitated and starts throwing up water. So rather than Orm and his fellow pure Atlanteans still going about their business instantly upon exposure to air, they need to get that water out of their system before doing anything else. This doesn't sound anywhere near as disgusting as the vomiting sequence from Blockers earlier this year, but it's nonetheless unsettling, not to mention scientifically sketchy. Oh well, the effort is appreciated.
For Arthur Curry, whose is half-Atlantean, half human, one would imagine that he doesn't have to put up with this messy biological trait. It also probably helps that he grew up on the surface, though as we saw in Batman v Superman: Dawn of Justice and Justice League, he can certainly handle himself in the water. This advantage will come in handy during the Aquaman movie, because although they're related by blood, Arthur and Orm will find themselves on opposite sides when the latter works to unite the seven underwater kingdoms to declare war on the surface. We knew that blood would be shed during the conflict, but now we can also brace for vomiting.
Aquaman dives into theaters on December 21, and as for what else the DCEU has in development, you can find that information in our DC movies guide.