The Water Prison is the third world visited by Henry Townshend in Silent Hill 4: The Room and is revisited with Eileen Galvin in the second half of the game. The Water Prison is a panopticon located somewhere on Toluca Lake; its precise location is unknown.
The moving turret system was probably part of the water prison's original features before the Order began to use it and consists of three concrete cylinders on top of one other than can move independently of each other. The Order used this system to dispose of corpses in rooms and light rooms using the holes in the floor. Henry must activate the lights and align the correct rooms in order to jump down to the cellar to find Andrew's corpse.
It is seen to the outside world as simply a disused water treatment facility shaped like a giant concrete cylinder close to the Wish House Orphanage. However, inside it is a brutal prison used as punishment to the orphans who disobey or are simply unlucky — Walter Sullivan frequently mentions in his diary of being put in the prison despite doing "good reading".
It is unknown how long the Order has been using the facility as a prison for the orphans, but given how effective the prison is at disposing of corpses through a sophisticated turning turret system, it may have been used by the Order for a number of years before Walter Sullivan's time there.
Andrew DeSalvo was a guard working at the prison at the time of Walter's incarceration, and according to diary reports, was considered the most feared and brutal guard of all. He seemed to take great pleasure in observing and watching the children, and he frequently beat and humiliated them. When Henry meets Andrew, he appears to be merely a shadow of his former self: nervous and concerned for his own life, yelling that "Walter is gonna kill [him]" even after releasing him. Andrew appears as an overweight, pitiful man, pleading with a boy (a young Walter Sullivan) for his life. The boy walks away with no sense of remorse, and soon after, Andrew is found drowned in the water with the code "18/21" carved into his stomach.
The reason the water prison is included in Henry's journey is because it is another location that played a very important part in Walter's life, albeit a very negative one. All of the locations visited by Henry are connected to Walter, implying the theory that Henry is indeed just a pawn in Walter's ultimate plan to open the door to his "mother", and Henry just happens to be in the wrong place at the wrong time. Similar to Silent Hill, Henry visits Walter's memories and fantasies, as Harry Mason visits Alessa Gillespie's memories.
- It is rumored there's a death chamber behind the kitchen, and the cooks take meat from dead corpses and cook it. The 1F Surveillance Room Report calls it an "interrogation room".
- It is possible Walter has never seen the "torture room" himself and the room that Henry visits might be Walter's fantasies as it lacks details, similar to surgery rooms in the hospital world, and it has some nonsense elements like giant saws and medieval torture devices.
- Below the Water Prison, there is a Spiral Staircase room possibly hinting about the prison's torture room behind the kitchen.
- The moment when Henry and Eileen descend from the sky might be an allusion to the banishment of Adam and Eve from the Garden of Eden for eating the forbidden fruit from the tree of knowledge of good and evil. Eve was the first who ate the fruit and then gave it to Adam. Many of Walter's diaries are written on trees and only Eileen can read them, giving Walter's "knowledge" to Henry. The forest world could represent the corrupted Paradise and Jasper's ghost could represent cherubs, an angel with a flaming sword who God assigned to guard Paradise.