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I posted this on the deco stop in November. It had to do with a trapped diver during a recovery, but the meat of what you are asking about is here.
I have raised at least a hundred cars on OC (sometimes using and EXO or bandmask on scuba with a comm line), as well as 20 or so boats (Up to 46") after Katrina. I am not sure what would cause a diver to become trapped, and I don't mean this to sound as if I am saying anyone is doing anything wrong.
The reason I say that is I have a system that keeps me out of harms way even if there are cars stacked on each other. When I locate said vehicle I shoot a bag and tie or clip my reel to the car. I surface and then return with a small rope attached to a float. I removed the bag and reel after attaching the new heavier line. This is my downline as well as marker if the chain, cable, or strap comes loose. I prefer to use a strap for attachment to the vehicle. Chain is heavy and tends to get lost and come loose. I use a large float ball to attach the tow hook and cable. I swim to the downline with a floating rope that is attached to the ball. At this point I descend and using the vehicle for leverage I pull the rope until it is straighht up. This is always in no vis water. I secure the rope, leaving a small amount of slack and let the rest float up and out of the way. I go up that line and remove the hook and cable. I pull the ball to the downline and using it, I head down. I usually shackle the eye of the hook to the strap with a shackle that is already in place on the strap. If I an on an EXO or bandmask I will tell the operator to pull the slack in and wait to make sure I have a good attachment point. If not I will surface and have the operator start pulling. The markerball will tell you what the car is doing and usually you can get out of the way. I have never been in more than 40' of water for a car recovery so surfacing and descending on the line is not a problem. I guess what I am saying, is this: Here is another option if you are not using this one. It has been very safe for 10 years now, with no close calls, and maybe it will eliminate the issue of a trapped diver. A contingency plan for a trapped diver still never hurts. A large bag would still have to be secured, airlines run to it, a cradle or attachment point to keep it from just rolling around and other options would make this a time consuming effort. The best way is always slow and methodical keeping our divers out of harms way from the get go. Hope this gives another perspective. On another note my team (police dept) only recovers vehicles if there is a body inside or it is a crime scene. If it is only stolen and not going to be processed or accidentally submerged the towing company or insurance company is responsible for the diver, which fortunately here is me. Mark