Checking out the HEED-3 website, I ran across the following testimonial from a diver involved in a vehicle extrication following an accident similar to the one contemplated in this thread. It shows the difficulties and complexities involved in such a situation:

"On February 12, 2002 myself and two other divers from my rescue company responded to a single motor vehicle accident that ended in a water rescue and subsequent recovery effort by our dive crew. Without notice the aft most heaving cable broke loose from its turnbuckle and like a pendulum struck my tank just above the valve stem shearing the 1st stage regulator from the tank valve and pinning me against one of the dock pilings between the van. I managed to wrestle my spare air bottle free. We carry our Spare Air bottles in a piggyback sleeve mounted on our tank closest to our back. Not knowing where my partner was due to the disorientation and shear terror of the blow and quick trip through the murky water and no communication possible without my mask in place, I decided to head up in case the second shackle let loose and pinned me to the bottom. Being @ more than 100 feet, I didn't know what my air supply would be like for my assent and I was breathing like a vacuum cleaner, sucking back air like a rookie! Despite a couple of bruised ribs, stitches and minor concussion, I came though this ordeal relatively unharmed and ALIVE!"

Note that this experienced and qualified diver was "breathing like a vacuum cleaner" in this situation....
Geezer in Chief