The Devil's Rock

On 5 June 1944, commandos are sent to the Channel Islands on sabotage and distraction raids, to draw Hitler's attention away from Normandy. That night, two New Zealand soldiers, Captain Ben Grogan (Craig Hall) and Sergeant Joe Tane (Karlos Drinkwater), paddle in their Klepper canoe to Forau Island, landing on a beach covered in anti-personnel mines and tank traps. When they leave the beach and head inland, they begin to hear distant screaming and gunfire. They approach a German fortification and hear what they think is a man being tortured. They climb down into a large gun pit and place explosives on a large artillery gun but are disturbed when a German soldier (Luke Hawker) runs out of a tunnel pleading for help. Grogan stabs the soldier in the back of the neck and kills him. They hear a woman screaming, and Grogan decides to investigate while Tane remains outside. However, when Tane hears a gunshot he also enters the bunker to investigate. While looking for Grogan, he discovers a book of black magic and, distracted by its contents, is killed by an unseen assailant. Grogan, unharmed, later discovers Tane's body but is immediately knocked unconscious by the Nazi.
Grogan wakes and is briefly tortured by a Nazi, Colonel Meyer (Matthew Sunderland), who wants to know his mission. During the interrogation, Grogan hears a woman screaming from another room. He eventually escapes and chases Meyer into the tunnels, shooting and injuring him. When he follows the sound of the woman's screams up to a room covered in occult symbols, he discovers that the woman is his dead wife, Helena (Gina Varela). Meyer enters the room and shoots Grogan in the leg, then shoots Helena in the head, apparently killing her. Grogan attacks Meyer, who explains the woman is a demon, summoned up from a book of black magic found on the island. Meyer proves this by offering her the leg of a dead German to eat; she changes into her true demon form as she eats the leg.
After Grogan removes a bullet from his abdomen, Meyer passes out. Grogan searches him and discovers a page torn from the book of black magic in a small pouch worn as a necklace by the Nazi. Grogan keeps the page after replacing it with another from the book. Soon after, Meyer recovers and explains the demon is a shapeshifter and a weapon the Germans plan to use against the Allies. He also explains that it is confined to the island because it cannot cross moving water. However, Meyer now realizes the demon poses too great a threat to the world. Meyer offers to give the book to Grogan if he will help him escape from Germany. Meyer then persuades Grogan to help him perform a ritual to dispel the demon back to Hell. Meyer, believing he is protected with the incantation sheet from the book, betrays Grogan at the end of the ritual.
As Meyer reveals his true intent to use the demon for the Nazis, Grogan overpowers Meyer and throws him to the demon. She brutally kills Meyer while Grogan survives, protected by the incantation he has taken from Meyer. When she tries to convince him to take her with him as Helena, he tells her that she could never replace the real Helena he knew, taking the opportunity to chain her up again. Unable to complete the ritual alone, Grogan takes the book and leaves the demon behind, to prey on any Germans that come to investigate; he explains to the demon that he intends to come back when the war is over to finish the ritual and to banish her forever. He leaves the key to the chain within reach as he leaves the demon in the bunker. He steps onto the beach, buries the photo of his wife Helena he kept with him, then looks up and sees planes flying overhead and an armada heading towards France. D-Day has begun.
One Nazi does fall prey to the demon, who masquerades as a German-speaking woman, Nicole, during a coda in the closing credits.

Question: Why is the demon confined to the island?
Answer: water