Once upon a time, Gulf Air lost a Skyvan in the Gulf, not very far from Das Island, following a double engine failure.
The pilot did a successful ditching, and was picked up, along with his only passenger, from the top of the aircraft by a helicopter which was in the area and heard his Mayday. The aircraft sank shortly afterwards. The helicopter pilot had obtained a very good fix on its location; plus/minus 100m, say?
We wanted to recover the aircraft to find out exactly what had happened, especially the fuel cross-feed settings. The seabed was flat, sandy and quite shallow; about 30m is my recollection, but I can't remember.
We hired an oil industry service vessel equipped by Decca and capable of finding almost anything made of metal on the seabed, down to a large wrench.
We paid for 10 days searching by that expensive piece of kit, and they found nothing. After that time the magnesium in the engines would be trickling on to the sand, so we gave up.
With that experience, I have to say that finding any part of MH370, let alone the FDR and CVR is unimaginably difficult by comparison with our search, with the position uncertainly, great depth and seabed topography. Any success will be the result of very, very intelligent guesswork, a lot of experience of the ocean, and a huge dose of luck.