Breather

(Contained, doable in a small studio mock up with NO water involved: like 'Buried' but cheaper)

Geographer ANNA (30s) has cadged a dive in a one-compartment, tiny sub that's now suffered catastrophic systems failure. Technician POWERS (male) and pilot SPICER (female) put the problem down to the outdated vessel. As oxygen depletes, in confinement lit by a Zippo, a torch, LEDs and a monitor, conflicting values boil. A patch to the mothership, via the umbilical cable, has WALTERS (male) revealing that all the topside crew is ashore due to food poisoning. Suspicions mount when the emergency oxygen is found to be non-existent. Anna identifies a 'red tide' (man-made dilapidation of the seabed). She couldn't live with herself if she didn't get the issue exposed. But there's no mobile signal at this depth.

Walters turns out to be Anna's exe. He's rigged the failures and absenteeism to facilitate revenge on Anna for ending their relationship and enforced by her wanting to publicise the marine damage she's discovered, which would adversely affect his company. He raises the sub away, then lets it plummet. Coming to from concussion, Anna finds Powers has murdered Spicer, thinking it's situation hopeless and wanting to die having sex with Anna. He swallows a tiny capacitor from the circuitry.

Pandering to Powers's S/M proclivities, Anna strangles him with her phone cable. Capacitor retrieved; she susses a rewire that allows her crank up the sub manually. Pressure causes a viewport seal to leak. Rising, Anna demands $10 billion of the company's profits syphoned off for others to put into green energy projects, or she'll blow the gaff. Walters gets the green light on the deal. Anna must thwart a reanimated Spicer. Surfacing under stars, Anna cries - as if rebirthed. Walters fires a harpoon through the viewport, piercing her heart. A helicopter heard returning, Walters retreats. With a phone signal achievable now, Anna sends her news viral. Her smiles freezes.