Dark Waters

A body...a dead body...will sink faster if it's face up in the water. Residual air in the lungs helps keep a body afloat. A body that's face up will begin to fill with water and sink more quickly; a body that's face down has no way for that residual air to escape.

Unless, of course, a few holes are poked through to the lungs. Sometimes you just have to help Nature along.