The shark effects are realistic enough to suspend disbelief and the attack scenes are both shocking and gory to please horror fans while still not crossing beyond the PG-13 territory.
Lively lives up to her name with an unmannered performance. The whole film rests on her shoulders and requires the audience to care about her well-being. It's a testament to her that for 90 minutes, they do. Her calm resolve when sewing up her leg, speaking to herself as if she were a patient in an ER, her devastated reaction when others are harmed, as well as her kindness to other characters, builds a protagonist for whom audiences can root.