Women using Twin Hose Demand Valves
As the diver breathes, her air is delivered through one tube to her mouth. When she exhales, the stale air is exhausted through the other tube and out through the regulator on her air tank.
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The girls get a little assistance with their air tanks
Don't you just love what she did with the snorkel in the center pic.
What's the one on the left wearing on her back? She seems confident it will give air.
Some basic underwater drills with a twin hose