.. it seems wikipedia already has a list of unrecovered flight recorders, though how complete it is is anybody´s guess:
List of unrecovered flight recorders - Wikipedia, the free encyclopedia
The boxes have been proven crash proof, fire proof and water proof. Making them easier to find in deep waters should be an immediate priority. Of course, retrofitting all airliners with improved pingers, for instance is not an easily accomplished task but what else is there?