It's the only room in the house that has a lock.
Water is very evocative.
If Psycho made explicit associations between death and the shower, other movies before and since have found death in the bathtub. The bathroom is a terminal destination for the expended fuel that keeps us alive, and occasionally for living beings themselves. It's the end of the line, a place where life itself may be flushed away. Bathtubs and bathrooms are often the sites of suicides, onscreen and off, of people choosing to shuffle off their designer clothes along with their mortal coils, and leave the world naked, the same way they came into it. We go in with Jack, who discovers a voluptuous naked woman in the shower.