Just my 2 cents, but has anybody noticed this ?
I replayed the demo several times and noticed that, if you throw a body part of any of the corpses littering the flooded cellar into the water, the water lurker goes after it...
...and seemingly devores it !
The good thing is, if you throw one or two limbs of the poor dead saps as far away across the room as you can, the lurker will follow it, get fed, and leave you alone. Pretty much like the mutated sled dogs and the beef jerky in
Overture...
"You... silly Billy !" (Clarence, Penumbra : Black Plague)