none of this equipment would be capable of finding aircraft wreckage fully submerged in deep fine sediment (think, the Mary Rose)
Deep water sediments are not to be confused with the silts and muds of near-coastal environments. The latter are mostly riverbourne clays with a large organic content, and indeed will not support any heavier object, it will sink in (though MR was covered with silt after sinking, the exposed parts eroding away). Deep marine sediments on the other hand are primarily the shells of tiny marine organisms, settling on the bottom in a fairly firm compacted layer (turning into chalk or limestone given a few million years). Larger aircraft parts with a big enough surface area to be detected will drift down slowly and settle on the top of this sediment. Several hundred years need to pass before they would be covered. Heavier items like engines will sink in somewhat, but still the majority of their bulk will remain above the sediment. AF447 remains were found in a similar environment and they all rested on the seafloor.