Sunny, whilst a rec diver can go to those depths, I doubt your average one would be able to recover the recorders or rig up hoisting lines. But I get what you are saying- for a pro salvage mob it isn't (wasn't, obviously) that hard. They should have done it asap.
Part of the reason why they decided not to recover the recorders initially was that nobody died. Which leads me to think, if a Boeing or Airbus ditched somewhere in similar circumstances, in about the same depth of water, and nobody died, would the atsb/casa still leave the recorders on the sea floor? I highly doubt it!