Place the water, sugar, tofu, cornstarch, oil, and salt in a blender, and process until very smooth. Pour this mixture into a saucepan. Place the saucepan over medium-high heat, and bring the mixture to a boil, stirring constantly with a wire whisk. After the mixture thickens, reduce the heat to low, and continue to cook, stirring constantly with the wire whisk, for 1 minute longer. Remove the saucepan from the heat, and beat in the lemon juice. Pour this hot filling into a fully cooked and cooled pie crust. Cover the top of the pie with a sheet of waxed paper, pressing it against the filling very gently to prevent a "skin" from forming. Chill the pie in the refrigerator for 8 hours or overnight.