Blake Lively in The Shallows.

Blake Lively, whose best role until now came when she played the secretary in that Saturday Night Live “Potato Chip” sketch, is terrific in The Shallows as Nancy, a medical-school dropout who goes to a secret beach in Mexico in the wake of her mother’s death.

Nancy sets out for a day of surfing and reflection in what she thinks is a completely solitary setting (with the exception of a couple of other friendly surfers). However … turns out there’s a huge great white shark there, and this is its part of the planet—and no trespassers are allowed, even if they are as pretty as Blake Lively.

As shark movies go, this is a good one, with decent CGI effects, a couple of tense shark attacks, and a constant level of terror that never lets up. The only thing keeping this from being “very good” rather than “decent” is the ending, which made me laugh when I should not have laughed. Despite this big flaw, The Shallows is one of the summer movie season’s better offerings in terms of fun.

Note to producers: Please don’t make a sequel where the shark’s offspring follows Blake Lively to a vacation resort, Jaws: The Revenge style. You’ve made an all-time Top 10 shark movie. Quit while you are ahead.

The Shallows is playing at theaters across the valley.