Is it shark week already? Maybe not, but Johannes Roberts’ “47 Meters Down” is based on the premise that “Jaws” is always in season. In a scenario reminiscent of last year’s “The Shallows,” in which pretty solo swimmer Blake Lively was trapped on a rocky growth some distance from a deserted beach by a predatory fin-bearer, dim-bulb sisters Lisa (Mandy Moore) and Kate (Claire Holt) think it would be a fine idea to energize their Mexican vacation by descending into the deep in a cage to observe the ever-chomping critters up close. Bad decision: the cage becomes detached and sinks to the bottom, where the women find themselves circled by the jaws of death—and quickly (though for this viewer, not quickly enough) running out of oxygen. Can Captain Taylor (Matthew Modine) and his crew in the boat above get help to them in time?

Lisa and Kate are in Mexico, we’re told, because Lisa’s boyfriend dumped her on the eve of what was supposed to be a joyous trip together, prompting Kate to step in to use his ticket. It’s she who suggests the shark cage adventure as a means of bolstering Lisa’s spirits after it’s suggested by a couple of handsome locals. Captain Taylor’s boat (and the rusty cage) might not look state-of-the-art, but he outfits the damsels with modern scuba gear—including microphones that will allow a steady stream of conversation and gauges to disclose the cage’s depth and the amount of oxygen remaining in their tanks—before lowering them gingerly into the drink.

Unhappily, after just a few minutes of fun—including the first of many “gotcha” moment when a shark suddenly appears out of the darkness—the winch chain comes loose, sending the trapped women tumbling to the ocean floor—they land at the titular 47 meters. They squeal in panic and try to comfort one another; Kate will also venture outside the cage intermittently to restore the communication link with the boat above as Lisa—the scared one—whimpers and awaits her return.

Surprisingly, the sharks appear rarely during their ordeal, abruptly showing up whenever Roberts must liven things up with a jolt. Such interventions are certainly needed, because the dialogue becomes terribly boring, except for the periodic instructions from the captain that are all too obviously designed to prepare for later plot developments. When the beasts do show up (in pretty good CGI), however, all they really offer is a brief shock moment, because—with a single exception—they seem to be pretty inept at clamping down on a prospective victim. They come rushing out of the murky depths—and murky they are—only to miss their prey by inches in almost every instance. (The exception, moreover, is really a cheat. One doesn’t want to reveal too much, but most viewers will probably feel let down by the ending, which strains for gruesome cleverness but instead is likely to elicit groans, even though Taylor’s repeated warnings have telegraphed it.)

Along with cinematographer Mark Silk, Roberts does manage to convey the extreme darkness of the environment in which Kate and Lisa find themselves; they can see very little, and neither can we. The effect is to create a strong sense of claustrophobic unease, but the director—whose last film was the soggy Indian-set horror opus “The Other Side of the Door”—doesn’t use the atmosphere especially well. The long sequences of Kate swimming about in the murky morass quickly become dull, especially since the dialogue is so banal, explaining why Roberts and co-writer Ernest Riera resort to plot turns so hokey that they actually draw guffaws from the audience—like one in which the cage is reattached to the boat with a rope rather than a chain, with predictable results.

Within the confines of the script, Holt and Moore do their Dumb-and-Dumber routine well enough, but never manage to engender a great deal of audience sympathy. Of the others only Modine is at all notable; his actual footage is limited, and in the later stages he’s present mostly in voiceover, but that at least gives an actor once so promising in films like “Full Metal Jacket” a welcome degree of anonymity as his lines get increasingly ridiculous, even though they are necessary to prepare the way for that deceptive finale.

In an era that has descended to “Sharknado,” any new attempt at a sharkfest needs to be really distinctive to carry much impact. “47 Meters Under” isn’t, and as a result comes across as a fairly toothless entry in the “Jaws”-inspired genre.