Athletic Jason

Answer: "Aquaman" director James Wan commented on this when a question was posed about if Aquaman is bulletproof, how can he get tattoos. "Bullets (or whatever else) will penetrate and break [Arthur's] skin. He's not a man of steel. He just has really dense mass/muscles. At least that's what Geoff Johns and I talked about." In addition to his advanced resilience, he also has advanced healing abilities. Geoff Johns is a comic book writer whose work includes Aquaman, and he was President and CCO of DC Entertainment at the time. In the comics, Johns has shown Aquaman to be vulnerable to bullets.

Bishop73

Answer: Just because a bullet won't kill him doesn't mean he'll get hit by bullets if he can avoid it. It would still cause pain. He also isn't bullet proof, he is merely bullet resistant. Larger calibre bullets would hurt him.