The first under-ice explorers discovered an annoying trait of normal regulators: The flowing gas in the near-freezing water drops the temperature enough to ice up the first stage. This freezes it open, causing a runaway freeflow.
Fortunately, there's a solution: Add a chamber to the first stage to surround it with silicone oil or other non-freezing fluid. The chamber has a diaphragm or piston that transmits water pressure to the oil, so the first stage does its job with minimal freeze risk. An added benefit in normal diving is that the first stage never gets salt water inside, minimizing maintenance concerns.
The downside? Sealed regulators cost a bit more to buy and a bit more for annual servicing.