If you expect rampant plot twists and character development, you must not see this type of movie often. It is what it is and it knows that. Despite the only real mystery being which of the girls survives, if any, or perhaps both, the movie actually manages to provide a refreshing twist.
That may sound bold to say when talking about a powerfully simple shark attack movie, but take some time to think about it. The twist is secretly ingenious and explains away many of the ghastly errors in believability.
I'm surprised to find myself defending a movie that I didn't really like all that much. But I am. It's not that movie doesn't deserve criticism. It certainly does. The acting performances earn failing grades across the board. The dialogue is laced with embarrassing over explanations and repeated lines, most of which involve warnings from the captain about the bends. Other times this takes the form of the girls obnoxiously narrating the menial tasks as they carry them out. Once or twice might have been fine, but it's tough to stomach after about the 10th time or so.
My thought on this is maybe the filmmakers knew that it was too dark to see what was going on for most of the movie, so they made the girls say aloud everything that they did. This solution makes about as much sense as going on a five-mile run after knowingly drinking spoiled milk. If one problem is easily avoidable, avoid the problem. Don't attempt a remedy that almost certainly won't help the situation.
All that said, I defend the movie because I think it executes its mission reasonably well. I shifted to the edge of my seat and felt uneasy on many occasions. The movie managed to scare the audience and it did so with fewer jump scares than expected.
Instead, the movie built to moments that would normally climax with the infamous jump scare then simply opted not to deliver one. The goal is still achieved. The moments still induce a sense of dread from the audience, but without the cheapness of frightening viewers by essentially saying "boo." I'd like to see other movies emulate this tactic.
My advice: if you are going to watch this movie, understand what you are getting into. The movie likely won't surprise and that wasn't its intention. Be reasonable with your expectations. Accept the movie's strengths and weaknesses. Empathize with how terrifying the situation would be if it were you stuck in the shark cage on the ocean floor, and don't hesitate to laugh at the movie's weaknesses.
Do those two things, and you may walk away satisfied. If you are unwilling to do that, stay out of the water.