So let's say that they by some miracle survive implosion without getting knocked out, and have lungs full of air - well, that air would still compress by a huge

So let's say that they by some miracle survive implosion without getting knocked out, and have lungs full of air - well, that air would still compress by a huge amount. And with negative buoyancy, they would sink. Pressure at 2000 feet is still a bit over 400 PSI.