What is the bigger tragedy - having several average-looking people killed by a shark, or having Blake Lively's endless legs scarred by one? Such is the question posed by The Shallows' director Jaume Collet-Serra, who pitches the gorgeous actress against a ruthless shark - and his own determination to figuratively reduce her to body parts.

Lively plays Nancy, a medical school drop-out experiencing an existential crisis following the death of her mother. In an attempt to feel closer to her late parent. Nancy travels to a secret beach in Mexico where her mother once surfed.

The Australian beach used as a Mexico-substitute is stunning, and Collet-Serra uses slow-motion shots of Nancy surfing (with Lively's face superimposed onto surfing double Isabella Nichols) to allow the audience to enjoy the pristine beach and luminous aqua waters. However, his editing is often clumsy and amateurish, cutting to overhead helicopter views when he should be showing us Nancy's POV, and awkwardly switching angles to make her physical position unclear.

One aspect of the director's focus that never wavers is his appreciation for Lively's breasts and backside, which he lingers on so greedily that he gives the shark a run for its predatory money. Lively battles through this gaze, proving engaging when she's bitten by the shark and trapped on a rock. Her medical school expertise makes her competent and level-headed as she's forced to MacGuyver a suture on her bloodied leg - and the presence of an injured seagull (either a Wilson-like projection or her dead mother, depending on your view) allows her an outlet to talk through her survivalist thought processes.

But as the third act transforms her from clever to a superhuman psychic superhero and the shark from a carnivorous fish to a scheming assassin hellbent on revenge, the average B-movie tension is replaced by toothless lunacy.