Here's a good graphic of the search region, around 43 deg S and 90 deg E, IIRC? The jagged yellow line is the South East Indian Ridge, the join line of the Indo-Australian and Antarctic tectonic plates.
If the aircraft ended up in the flatter regions of the S.E. Indian Ridge, it could be up to 4500M deep - if it just happened to lodge on top of one of the underwater volcanoes, it will more likely be a modest 2000M down.
File:Seir.png - Wikipedia, the free encyclopedia