I think the face-down, head hanging posture is due to flotation in a large body of water, the weight of the body pulling the chest, abdomen and heavy head face down, while the lungs keep the body from sinking. If the body died in a bathtub, there probably wouldn't be room for the body to turn over, but if it was a short body and a long tub, the unsupported head might well sink down, towards the back. Tubs can be tricky. There was a case in Britian years back, 'The Brides in the Bath Murderer' (Google
http://en.wikipedia.org/wiki/George_Joseph_Smith) the police suspected a man of drowning a series of wives in bathtubs. They drowned face up, but apparently it was simple to do if you knew the method.