chu chu, the general sequence for searches of wreckage where one doesn't know the precise location, is to use side-scan sonar to sweep where one thinks the wreckage might be. If there is a hit, i.e., an area worth investigating further, then a submersible is sent down to photograph. If the visuals confirm that's the object one's searching for, then more extensive photography is done to see whether objects that one wants to retrieve can be located. The ultimate phase would be retrieving any located objects.
A straightforward and laymen's description of the technology and the challenges can be read here, on the search for HMS Hood (and the Bismarck as well). Of course, this involved large ships resting at great depths on a relatively flat bottom, with a fairly good notion of where they were.
Channel 4 - Hood v Bismarck - The Search - Method
Channel 4 - Hood v Bismarck - The Search - Equipment