I haven't seen the movie, but I do remember the "incident". Empty tanks float. I would imagine they would want as much bouyancy as possible. Inexperienced DM (or an idiot. Gringos in should equal gringos out all the time, no exceptions). I was on a drift dive in Cozumel a number of years ago where we did lose a diver. Actually, she lost us. Her dive buddy (and friend) couldn't clear so she saw her bud back to the boat and then decided she could catch up to the rest of us. She couldn't find the rest of us so just cruised down the reef, out into open water, down to over 100 ft. When the boat picked us up, she wasn't there and her friend totally freaked. We cruised the reef for half an hour before the boatman started checking the other boats, and we did find her with another party. She was pretty freaked out also. Not a good experience at all.

Been there, done that, the washing machine ate the T-shirt