This might sound like the dumbest question - they had lots of difficulty locating the sub and there were lots and lots of crews from around the world. Then there's the implosion, and now there's debris which are easily located and (very quickly?) identified to be that of the sub. How? I'm guessing it's based on the radars these recovery tanks use, in that they can be tracked as they float around?
I saw a graphic the other day about pressures at various levels under the surface of the water - even very close to the top of the sea level, a cork on a champagne will get popped back into the bottle. It's just very hard to even comprehend the kind of pressure the sub was experiencing at that level so far down.