|violence toward animals||spoiler, yes|
|X was the real Y||It turns out humanity was the real malicious shark|
I love movies with a single character (or almost, in this case)
I also really love horror movies where a single protagonist is trying to escape a singular threat. Technically, there are a few supporting characters (who, spoiler, get jawsed), but they barely appear and their deaths aren't lingered on. This movie is just a known threat, and an attempt to escape that threat.
And finally, I really enjoy horror movies without supernatural elements. In this case, the shark is unrealistically malicoius, but not in a way that detracts from the film.
Parts of the ending of this got a little silly, but I liked this a lot. I think Blake Lively did a great job in this.