[q=MJN Seifer]The water changes depth when Cloud kills Aeris

Also she moves somehow AFTER she dies.[/q]Cloud didn't kill her.

Also, the water doesn't change depths. There's a shallow part to that pond, and a deeper 'hole' in the middle of it. Cloud was standing in the shallows, and lowered her body into the water, where she sank into the deeper part.

She didn't move after her death; Cloud was cradling her body, and moved her. Also, Jenova.LIFE is pretty huge, so its tentacles and limbs probably pushed her around.