Deliberate mistake: Spoilers! When Black Manta is defeated, he is knocked unconscious and tumbles down a rock face after his helmet explodes on his face into the waters by the shores of Sicily. These currents and waves at best would have carried him to the rocks or the beach, leaving him on land or near it. However in the after credit scene he's somehow found out in the middle of the ocean on a broken drift wood. He would not have been carried back out to sea like that.
Factual error: An M203 round arms at between 14 and 27 meters, and produces a ground burst that causes casualties within a 130-meter radius, and has a kill radius of 5 meters. Even if it did work all 3 would be dead. (00:18:35)
Continuity mistake: Near the end, during the final combat scene between Aquaman and King Orm, the gigantic whirling propeller blades in the background noticeably change rotation rate: At the start of the battle, the blades are turning rather lazily; but at the height of the battle, the blades are spinning rapidly, a virtual blur in close-up shots, drawing a steady stream of sparks from Orm's helmet when it touches the propeller. When Aquaman defeats Orm moments later (and their mother, Queen Atlanna, appears), the huge blades are spinning slowly again. I'm not talking about during dramatic slo-mo action effects, but during the normal-speed action. (02:04:50 - 02:07:10)
Other mistake: Artistic license via CGI physics is taken for all the characters in the water to keep their hair from ever flowing over their faces, no matter how long or how the currents might affect their hairstyles. When characters emerge out of the water, their hair remains nearly unaffected.
Continuity mistake: After Arthur puts his father in the truck after the bar, we see Mera emerging from the water from the left of screen, right of the truck, off to the side casually walking up. When the camera changes we hear a splash and suddenly Mera is on the dock behind the truck dripping water. This is showing that she was walking up from the side, suddenly went back into the water, only to jump back out again at a different spot on the dock.