Here's what happened on a dive off Cozumel in 2002. We were doing a wall dive. I was at about 90 ft. when on of the divers from our boat drifted downward past me. She was feet-down, holding the deflate button down on her BC control with her back toward the wall, just looking out into the blue. The divemaster was above me doing a critter show with an arrow crab and two other divers from the boat. I looked back down and she was still holding the deflate button down and the control above her head, no bubbles from the BC. I dumped my own wing air and went head-down as fast as I could until I caught her. I grabbed her tank valve and began swimming upward as with as much effort as I could manage. We stopped drifting downward and as I powered the two of us upward I pulled the control from her hand and added some air to her BC. She was wearing at least 25 lbs. of lead. Gradually she figured out what was happening and began swimming up with me. At about 135 ft the divemaster caught up with us. I held on to her rig until I was certain that he had control of her. My computer showed 184 ft. I had about 900 psi left. I made an ascent slow enough to keep my computer happy, hovered at about 15 ft. until I was amost out of air then surfaced. The whole deal probably didn't take three minutes, but it sure seemed like a very long time. Boat driver didn't find me until after the DM told him where to look. I got to float in the ocean for about 45 minutes before they came back to pick me up. I was the last one back on the boat and one of three divers with no air pressure showing on my gauge. DM told me he had to share his air with her on the safety stop, so both of them were on 'empty', too. She never said anything to anybody on the way back in, but I got to dive for free the next two days.
DC