The rules of the pool water are immovable. There’s no outsmarting the water spirits or the powers within, or a loophole to be found that allows the family to stay as a complete unit. They swam in the water, Ray made a wish to get better, the water granted him healing, and now someone has to die. It’s a bold move by McGuire and company to embrace the bleakness of the water’s lore, especially in a landscape where most studio horror films try their hardest to maintain the nuclear family. Elliot Waller is the child with the “least potential” for a successful future as far as the pool spirits are concerned, and have made him their target. Even the matriarch of the house’s previous family — who knows what the pool is capable of as it took her daughter — tries to convince Eve that sacrificing Elliot is the better option for the family. It’s as if the spirits want to punish Elliot for not being unnaturally talented or powerful immediately, instead of being a kid still trying to figure himself out.
During the film’s climax, when it looks as if a possessed Ray is going to kill his son, the part of him within still fighting against the pool’s power breaks through. Instead of letting it take Elliot, he dives back into the water and sacrifices himself. He finally gives himself over to his kids, something he couldn’t do throughout his demanding career as a professional baseball player. The water, satisfied with its new soul, is done terrorizing the Waller family. But there’s no telling what could happen the next time someone takes a dip in its waters and happens to make a wish.