No vision
One of the most important pieces of diving gear you can have is goggles. Those plastic things that strap around your eyes allow you to see, which can mean the difference between bumping into a bull shark and swimming away before it notices you.
This person’s rented goggles had snapped off and floated away, never to be seen again. Luckily, they still had their wits about them, and they started knocking on their oxygen tank to draw attention from the other divers. They were left sightless for a mere 20 seconds before the others came to help. Again, only dive in groups.