When the kid at the start of the film is watching the found GoPro video, we see the surfer take a tumble on a wave and that's when the shark gets him. When the events are playing out during the film, the same surfer is in a completely different scenario when he's attacked.
The body of the drunk Mexican that collapses on the beach disappears when the two surfers return to the beach; it is never seen again for the remainder of the movie. One would think the two surfers would have discovered the partially-eaten body, as would have the boy kicking the soccer ball and the boy's father.
The height of the rock above the surface of the water varies wildly between shots and doesn't seem to have any relationship to the tide.
After she said "This is my headstart", she dove into the water and for a second or two, you would see the wound/cover/tourniquet switched to the right leg.
The body of one of the surfers ends up on the beach to be glanced at by the father of the boy kicking the soccer ball. It was not seen by the boy during the second shot of the boy kicking the soccer ball as the boy ran along the beach, and then turned to run back home.
When the chain breaks off the buoy it would not tip over. The chains are to hold it in place so it doesn't float away, not to keep it upright.
There are a few instances where the shark is shown backing up after striking an obstacle, something that is physically impossible for a shark. They would have to stop moving altogether and sink back to move backwards.
Nancy ignites a plume of whale oil coming off the dead whale with the signal flare. However blubber has to be boiled in order to release whale oil. It isn't naturally in a liquid state, and wouldn't leak out of the whale, into the water.
The bay is tidal and yet neither the whale, helmet, surf board or anything else drift from their positions.
There are a few moments in the film where the shark can be heard growling or snarling, sharks have no vocal chords or lungs to make such sounds.
Sharks don't take things personally. As long as the whale carcass was still around, a great white would have left a human alone. Whale meat is everything great whites need - fat-rich and full of protein. It would have focused on feeding on the whale carcass and pretty much ignored Nancy, who could have made the swim back to shore un-noticed. As it is inferred by Nancy, she has trespassed into the shark's feeding ground, it is no longer about food but about territory.
During Nancy's video call to her sister, her real facial movements do not match her facial movements as shown on the phone screen.
Although the film is set in Mexico, the seagulls (including Nancy's companion) are Silver Gulls, endemic to Australasia.
As all experienced surfers (as Nancy clearly is) would know, a whale carcass is almost guaranteed to attract sharks. Coming across one, a surfer would have hightailed it back to shore as fast as possible to avoid becoming part of a buffet. They would not move in for a closer look and hang about ogling it.
When she's timing the shark's course before swimming out to get the camera she says "32 seconds from the whale to the rock" ignoring the fact that the shark was only cruising. Sharks can swim in bursts of fifty feet per second.