The movie is very particular in setting up perimeters and rules. And making it as believable as possible, that in a modern time like this, someone would not be able to call for help (apart from the usual "no service" kind of cheat other movies do when it comes to Cell Phones). And if you buy into it, it works.
There's also the question why Livelys character goes there in the first place. Well even that is explained, so there is no doubt about motive too. What never is explained though, is not where the Shark came from or why it got there, but how it's possible that it's so bloodthirsty! We're talking Jason Vorhees or any other Slasher movie kind of mad. Maybe even more mad than that.
And while you could easily go, well it's a movie, it kind of breaks it's own rules there. Because everything seems logical (explaining mentioned), even the character and her ability to deal with (graphic) wounds ... but not the shark, who is really a monster if you come to think of it. But if you don't care about internal logic, like the view (no pun intended) and the thrill of it, you will have a good (scary) time watching this!
