They had plenty of time to prepare one and dump it there while they had everyone looking somewhere else. Even if they find a black box - or both of them - I won't be convinced at this point.
Why go through that much trouble???
Why dump it in 15,000 feet of water and go thru all the trouble to recover it since it is possible that you may not???
Why not just keep it on the deck, pour some sea water on it, and say look what we just found.
Who would know???
Ohh yehhh -- I forgot about the data inside -- the agencies of 5 separate countries will want to examine it.