If the water surface is 12 feet below the surface of the water, then the pool's depth is 12 feet.
It's important to note that it might have been a mistake or a typo in the initial question but depth is the distance from the top of the pool to the bottom of the pool, So saying "the water surface is 12 feet below the surface of the water" does not make sense since if the surface is 12 feet below the surface it would be considered as negative depth.