Dang! The guy I saw demonstrate it was only about 8" under… I, on the other hand, bob like a cork. One part of the training required that we tread water for a minute without using our hands (holding them out of the water for the instructor to see.) Most of the guys around me really struggled with this part, while I just kinda hung there. Conversely, when we had to do the underwater swim portion, to simulate swimming under burning fuel on the surface, I was pretty much screwed - I couldn’t keep myself underwater long enough. OK, end of hijack…
If you’re negatively buoyant, there’s no such thing as “8" under” or “2 feet under” being a neutral point. A mile is the same as an inch; you’ll just keep sinking unless you swim upward. It’s only for positive buoyancy that some part of your body might be above water to a greater or lesser degree.
ETA: Actually it’s worse than that. As you go deeper, the air in your lungs will compress, and make you even less buoyant. So there’s a runaway effect as you sink further.
I’m a natural swimmer, not afraid of the water. But I had a slight epiphany a little while back.
I was swimming beside a boat in a deep lake. I realized that all takes to drown is to lose that lungful of air and start sinking.
Pretty soon you’ve lost the surface, and can’t get another breath, and black out. Then you’re dead.
Scary thought.
…he said, resurfacing four years later. 
Didn’t this contribute to the drowning that killed Naya Rivera?