The science behind it is really interesting. I had always assumed that you'd get more buoyant as you went down and the pressure got higher; the thing is, buoyancy isn't about pressure as such, it's about relative density. Water is almost completely incompressible; even miles down, its density only increases by a few percentage points. But the main thing keeping humans buoyant is the air in your lungs, and your lungs are compressed as the outside pressure rises, making the airspace smaller and denser, until your average density is greater than the water's and you sink.