I have often wondered why these, valuable to the search and subsequent enquiry, items are not given some form of barostatic release. If so designed they could then automatically deploy to release themselves to the surface if the lost aircraft were to descend below a certain depth of water.
Probably because if they were still in an intact piece of the aft fuselage they would be required to unscrew themselves from their mounts, open the cargo door and swim to the surface?