The light blue/azure colour in tataka's current map also shows the thermocline.
The deepest part is about 600 feet and the shallower at about 350. Whilst the thermocline marks the difference between temperature profile is is also almost a 'hard' surface for buoyant objects. It is also the layer most affected by wind and wave action. Below that things remain faily constant.
IIRC sink rates for heavy objects would be about 40 seconds per thousand feet so for 10k feet it would take some 6-7 minutes. There would be sub-surface drift but it would be insignficant for an object sinking to the bottom.