Some additional comments - an airbag reacts in abt 1/25th of a second to a crash. Now let's assume the airliner slams into the sea at full cruise speed of 800 km\h which is waaay too fast. In 1/25th of a second it will have moved 8m or 24 ft if my calculations are correct. Most planes out there are considerably longer than that giving an ejectable black box in the tail plenty of time to eject to safety.
PS If you look at the condition AF447 debris was found in, some of the cabinets floating out there were almost intact, and they look no sturdier than IKEA furniture, so my guess is the terminal speed at impact wasn't that great.