Assuming so, are there other techniques that could be used to search? First thought to pop into my mind - magnetic anomaly detection (MAD). If there enough metallic content to make this a feasible proposition?
Many decades ago when I had some experience with MAD as installed in anti-submarine patrol planes, the effective range was around 1,000 feet. That may well have improved over the years but I would imagine that given the depth of the water in the vicinity of the AF447 crash, any MAD gear employed would have to be carried by some kind of submersible.