Plot hole: It shows two air tanks missing on the boat. Now obviously when they get back to shore and all the tanks are put back, there would be two empty spaces, meaning hey, we forgot two people.
Suggested correction: True, but who says they didn't alert the authorities or return when they eventually realised it? Even if they did, the two divers drifted and were being stalked by sharks. It was set up earlier in the film when they showed their flawed way of tracking who went into the water and returned, so it makes sense that they overlooked what happened. There's also a scene when the boat returns where an employee notices two missing tanks, so they did notice it.