This fluid actually exists in real life, but the only man who's tested it nearly bought the kelp farm. Not wanting to take his chances with a magical mystery liquid, Harris opted to hold his breath whenever he had to film in the suit. So when Harris is gasping and seizing up in panic as his helmet fills, he ain't acting.
As it turns out, nearly drowning was something of a running theme on the set. Another scene involved actors swimming without helmets, and since nobody in the cast was Aquaman, Cameron employed safety divers to stick life-giving oxygen into their faces. Then he told them to stay way the hell away from the camera, so if any of those scenes had gone wrong, the safety divers would have had a nice long swim to get to whoever's lungs were quickly filling with water. Oh, and one time a diver used an oxygen regulator incorrectly and pumped water into Harris' mouth instead of air and nearly killed him. Whoops-a-daisy!
It gets better. In order to ensure that nobody accidentally floated up to the surface, the actors were all strapped down with weights, meaning weak swimmers were screwed, and strong swimmers ... were also screwed. Yes, Cameron kept his actors in line with the same method the mafia uses to dispose of bodies.