Though the heroines make some terribly dumb moves in their bid to survive the ordeal, the script's clever reminders about deep-sea dos and don'ts will keep viewers invested in ascertaining the final outcome.
British director Johannes Roberts, with good work from cinematographer Mark Silk, allows the undersea action to unfold like a procedural, so that the details ratchet up the tension.
While the plot is a bit shaky in parts, the overall effect of creating needed tension and some outright, out-of-your-seat jumps of fright is quite effective.
On paper, the elements for suspense - sharks, rapidly depleting oxygen, a cage, risk of the bends - are all there, but on screen they add up to a whole lot of visual blah.
It's not a great film but, once the preliminary scenes on terra firma are over and we descend to a place we don't belong, it delivers an hour or so of sustained physical and psychological terror.
This low budget but highly-effective horror film is exactly the kind of movie that legendary Hollywood D-movie producer Roger Corman would have knocked out in the '70s: plenty of writhing female limbs, no surprises.