Treat the surface of the water as 0. The initial height of the weight was 4 ft above the surface of the water, therefore making it have an initial height of +4. If it travels 12 ft downward to hit the bottom of the pool, that means its height decreased by 12, so its falling is -12. To calculate the depth of the pool, use initial height minus drop distance, so 4 - 12. The depth of the pool is 8 ft.
