In the movie Aquaman, we saw that all those soldiers who came to attack both Aquaman and Mera, have water filled up in there suits. In one scene, when Mera broke one of the helmet of a soldier, he immediately went for the "toilet water", apparently can't breath air. But others, like Aquaman, Mera, Vulko, Orm etc can breath underwater as well as on ground.

Why is it so? As it was explained in the movie that when Atlantis went down, everyone got the ability to breath underwater. So everyone should also be able to breath above water as well, they were all "grounders" before Atlantis drowned?