What about high end rebreathers? The top of the line ones can sustain their oxygen cycle for six hours it seems that the divers are using oxygen canisters and there has already been one death due to a diver running out of oxygen.
Rebreathers are frail and bulky, and not good for cave diving (one of the divers involved had a special front-mounted rebreather to work around this). A lot fewer divers are trained to use rebreathers, too.
Cylinders are hardy and portable, and can be carried with one hand through narrow openings. They can be attached to lines, and stored along a prepared route for other divers to use during a single descent.
